Metric & Imperial Units in Blender
Blender provides three distinct types of unit that can be used to measure objects (distance) in a Scene; "None", "Imperial" and "Metric". Although relatively self explanatory in terms of what they represent, their use does come with some caveats the user needs to be aware of, which depends particularly upon the version of Blender being used.
With this in mind the following discusses where to find these settings and how they can be used within Blender to give content a real world measurement context. A basic understanding of Blender is recommended but not specifically required to get the most from the below.
The Units system ^
As briefly touched upon in the introduction, the distance and/or angle between two points or edges, or the area attributed to a given surface, can be measured relative to a choice of three distinct 'units of measurement', "None", "Metric" and "Imperial". Each 'type' of unit has its own set of options organised under Blenders main "Units" sub-system, which itself can be found within "Scene" Properties, and breaks down as follows;
"None" represents simple 'unit' based measurement.
"Metric" represents "millimetres", "meters" etc.
"Imperial" represents "inches", "foot", "mile" etc.
Design note: the 'unit' of Blenders default system "None", whilst similarly 'decimal' in nature to "Metric", has no meaning beyond its face value. In other words, by itself, a measurement of "1000" means one thousand units and nothing more. Whereas "1000" within the "Metric" system might be referring to any manner of unit types; is that one thousand metres, kilometres, nanometres etc.
In essence the way the "Units" system works is two-fold; 1) the unit type selected defines what each unit of measurement means (gives context to numerical values assigned to various properties and settings); and 2) arranges the appearance of the Grid in the 3D View to use a set of predetermined characteristics that alter the way major and minor cells are divided and scaled relative to objects in the scene and the unit of measurement used. What this means is that in selecting one or the other "Metric" or "Imperial" systems ("None" behaves as "Metric" but without the ability to set meaningful sub-selection or unit types), the grid is changed to display each cell larger or smaller depending upon the type of measurement selected ("Metric", "Imperial", "None"), and the unit represented ("Inches", "Yards", "Meters", "Miles" etc.) - the grid set to yards for example, displays smaller cells than would be seen under meters due to differences between the two unit types.
Design note: Blender's measurement system in it's entirety is largely composed of the 'unit of measurement' properties and the Grid displayed in the 3D View; the former sets the latter, with the latter acting as a visual representation of the former.
The default Scene when Blender is loaded showing "Render" properties selected - note that the actual options and settings visible will vary depending upon the version of Blender used (Blender 2.71 shown)
Clicking the "Scene" button reveals a collection of properties and options attributed to the scene itself, including "Units", which by default displays "None", meaning measurement is simply an expression of a 'unit' (the above is shown in Blender 2.71 but applies to Blender 2.56 and above)
In Blender 2.50 through to Blender 2.55 the appearance of "Units" properties differs slightly but the user is still able to select the appropriate type of unit system wanted at the click of a button
Selecting a Units option ^
As discussed above, after clicking "Scene" properties the "Units" sub-section presents three options; "None" (the default), "Metric" and "Imperial". When each is selected a different set of properties appear based on what units are to be represented and used;
"None" presents no additional options.
"Metric" includes additional buttons for "Degrees" and "Radians", a "Scale:" value and an option to "Separate Units" (checkbox).
"Imperial" includes additional buttons for "Degrees" and "Radians", a "Scale:" value and an option to "Separate Units" (checkbox).
Design note: when selecting "Imperial" the 3D Views Grid, when viewed in Perspective mode, tends to resize quite significantly because each cell represents a much smaller unit of measurement, "foot"; set to "Meters" each 'minor' grid cell represents a 1m x 1m area, a 'major' grid unit then representing a 10m x 10m grid of such cells; whereas set to "Imperial" each 'minor' cell is a 1ft x 1ft area, a 'major' grid then represents a 10 square foot area, which is significantly smaller than 10 square metres (this change/difference may not be visible in all versions of Blender).
Selecting "None" for "Units" no additional options are available because the unit itself, i.e. the distance its supposed to represent, is determined by Grid settings in "Transform" properties
Selecting "Metric" reveals an additional set of properties; a "Scale:" value, two options to determine how rotational values are to be treated and "Separate Units" (generally speaking these options are available in all versions of Blender but may be grouped or positioned in slightly different locations as is the case for Blender 2.50 through to Blender 2.55 - Blender 2.71 shown)
Selecting "Imperial" similarly reveals an additional set of options; "Scale:", the same two "Degrees" and "Radians" buttons and the "Separate Units" option (again as with "Metric", the position and grouping of these options may differ to other versions of Blender as is the case for Blender 2.50 through Blender 2.55 - Blender 2.71 shown). Note additionally the significant change in size to the Grid visible in the 3D View as a result of it using much smaller units of measurement, "Feet" instead of "Meters" (this change/difference may not be visible in all versions of Blender)
Default Object, Units and Grid size ^
Alluded to in the above, switching between "Metric" (or None) and "Imperial" results in quite a significant change to the Grid. This is largely because the different systems use quite distinct base units for measurement; for "Metric" this is "Meters"; for "Imperial" it's "Feet". In other words each minor grid cell is either 1 x 1 metres under the Metric system, or 1 x 1 foot using Imperial. Whilst this isn't problematic in of itself, the difference has an unintentional consequence on the default Scene Cube; switching from Metric (or None) to Imperial, it's slightly too big; going the other way, switching from Imperial to Metric (or None), it's too small.
Design note: this assumes Units are switched after Blender and the default Scene contents have loaded, but before switching between the unit systems.
By default the Scene Cube appears centred on the Grid as a 2 x 2 x 2 unit object, "unit" being the base measurement. When switching to "Metric" this same cube is measured as a 2 x 2 x 2 metre object - "Metric" and "None" directly correlate. In Imperial because the based unit is "feet", it should appear as a 2 x 2 x 2 foot object but it doesn't, its size remains relative to its original dimensions, now a 6' 6" x 6' 6" x 6' 6" cube (1 metre equals 3' 3", so the cube, originally being 2 metres, is 2 x 3' 3" or 6' 6" across). In other words its size doesn't (always) change when switching between None/Metric and Imperial to properly reflect the selected Units system. This is generally true of all content, which remains true-to-size irrespective of the unit of measurement set.
Design note: this is also true in reverse; when working in "Imperial", and switching to either "Metric" or "None", the opposite problem occurs where distances are then proportionally too small because of the up-scale in unit size; items then have to be re-scaled upwards to match the new, larger grid.
The upshot of this is that when starting a new project, clearing (deleting) the default cube and loading in a new one might be necessary. This places into the Scene an Object that's correctly proportionate relative to the underlying base unit of measurement. To add a new object, with the cursor over the 3D View, press "Shift+A" to access the "Add" menu options, select the object required (which will be added at the 3D Cursors location). Alternatively click the "Add" menu option in the 3D View Header and select an item from the list. Or from the Tool Shelf on the left of the 3D View, click the "Create" tab then on a button representing the required object.
Design note: when reloading the start-up file, unless "User Preferences" have been changed, the Scene that loads is set to use "None" units of measurement with the default Cube sized appropriately. Adding Objects loads them relative to the Cursor location so that may need to be reset or moved beforehand.
The default Scene showing the default Cube in transparent grey so the 'corrected' cube for Imperial measurement can be seen through it, highlighting the difference between the two - the red 'target' area indicates the relative size of the "Metric" base unit being "Metres"
Similarly the same objects shown as above with the red 'target' area indicating the much smaller unit for Imperial measurement, "Feet" - preference should be paid to clearing the file and loading in an appropriate object when switching, which Blender automatically resizes relative to the base unit used by the system
Shown in Perspective mode the white target area highlighted indicates the size difference measured in metres - note the 'unit' reference top left of the 3D View - "Meters"; note also the transparent cube representing the defaults fits exactly within the Grid lines indicating its 2 x 2 x 2 metre size
Using " Imperial" measurement, the central cube is the correct 'default' that should be loaded as it is also 2 x 2 x 2, but in 'feet' not 'meters'. Note the Grid, relative to the original object, clips, whereas the smaller object fits exactly within as should be expected
Units Scale/Size ^
Along side the establishment of each systems base 'unit', are each their respective "Scale:" values. What this does is allow different levels of measurement accuracy to be defined within the bounds of the aforementioned system base unit. In other words in wanting to measure 'Inches', the default value of "Scale: 1.000", which represents 'feet', needs to be decreased, for 'Miles' it would be increased and so on. For example when "Imperial" sets the default unit to "1 foot" with "Scale: 1.000" this means the value has to be changed so that;
0.083 = 1" (one inch).
1.000 = 1' (one foot).
3.000 = 1yd (one yard).
Design note: in Blender 2.50 through to Blender 2.56 the above unit values do not correlate with Blender 2.56 and after due to a differences in the way the system registers the base unit. In the earlier versions (2.55 and below) measurement was seen relative to "Scale: 1.000" being representative of 1 metre regardless of the unit system. In other words this means Imperial unit calculations are done relative to dividing metres, not feet, by the decimal version of an Imperial unit of measurement (see below).
When changing the "Scale:" value attributed to the use of Imperial Units, the relative size of the default Scene Cube object changes - although this is generally not seen as the object itself doesn't change insomuch as its "Dimensions:" (in 3D View Properties, "N") are altered to match the grid and unit scale
Similarly when "Metric" sets the base unit to "1 meter" and "Scale: 1.000" this means;
0.001 = 1mm (one milimetre).
0.010 = 1cm (one centimetre).
0.100 = 1dm (one decimetre/ten centimetres).
1.000 = 1m (one metre).
1000.000 = 1km (one kilometre).
Design note: when changing "Scale:" to alter the base unit from inches to feet, the dimensions of content in the Scene should change relative to the scale set. For example, using "Imperial" measurement the default cube is 2' x 2' x 2' because the base unit is "1 foot", change to a smaller unit, "inch", and the cube becomes 2" x 2" x 2" (inches). In other words the mesh doesn't necessarily change in any discernable physical sense other than its "Dimensions", its length, breadth and height changing in a way that corresponds to the different unit size/scale. This means the above unit examples equate the default cube to being;
The upshot of this change is to make sure the unit scale is set correctly before starting a project otherwise objects that might actually need to be large, but built relative to a smaller scale, may inadvertently have their respective dimension values resized to suit after-the-fact alterations to the Scale property.
Shown is the relative size of the default cube as seen when the "Scale:" value is changed to alter the unit of measurement - larger values cover larger distances, smaller values smaller distances. Again, when "Scale:" is changed the object itself is not modified insomuch as its general "Dimension:" are updated to suit the base unit System and type, i.e. "Metric" and "Metres"
To change the "Scale:" value simple left-click in the input field, delete any values as might be present and type. Press Enter to confirm the change (or click elsewhere on screen). Upon doing this the 3D View's Grid and other settings will update.
Design note: for Blender 2.50 through Blender 2.55 the base unit, "1.000", remains constant across unit types. Although this makes no odds for "Metric" because its fundamentally the same unit system as "None", it does however have an affect on Imperial because units, whether distance is to be measured in "inches", "feet", "yards" etc., have to be calculated relative to a base unit of metres not feet. This means the "Scale:" values (in "Units" properties) are different. For example;
1 inch = 0.0254 metres.
1 foot = 0.3048 metres.
1 yard = 0.9144 metres.
With that said, for those same versions of Blender scale is generally handled altering Grid settings, i.e. "Spacing:" and "Division:" (later "Subdivisions:"), rather than explicitly using the "Scale:" value available in "Scene" properties. Bear in mind also that Blender displays 'precision' numerical's by rounding up or down to the nearest whole number so values may appear, for the sake of display, incorrect when in fact they are not.
In Blender 2.50 through to Blender 2.55 the base unit of all measurement systems is "1 unit" (large blue/grey cube shown above), which translates directly to "1 metre". This subsequently means scaling, by Grid or "Units" properties has to be calculated relative to the decimal unit/metre when using Imperial measurements
Top down view showing the difference between base units. From Blender 2.56 changing to "Imperial" units of measurement adjusts the base unit relative to being "1 foot". This means 'inch' and other Imperial measurements can be calculated properly relative to Imperial units (although calculations are still required to be carried out in 'decimal')
View Properties & Units ^
Secondary to "Scale:" in "Scene" properties are a set of options available in "Display", a subsection of the 3D View's "Properties" tool shelf overlay. Here three settings allow for a slightly different way to control Grid sizing; "Lines:", "Scale:" and "Subdivisions:". Having said this however, the availability of the options depends on the "Units" type selected - when using "Metric" and "Imperial" measurement "Subdivisions:" is greyed out, although still usable despite both unit systems typically relying on predefined cell sizes. To access the options, with the mouse over the 3D View, press the "N" key, or from the views Header menu click "View » Properties". Once open scroll down to the "Display" sub-section to view/edit the available options (click the small triangle to the left of the heading to expand if not initially visible).
Design note: although "Subdivisions:" is greyed out to indicate its generally non-functional, it does work. This means that aside from using "Scale:" in "Scene" properties to set the base unit, "Subdivisions:" can still be used to define the grid for better display (and use) of the unit being used. The Grids appearance differs slightly when displaying this in Perspective and Orthographic modes.
Additional options for unit management can be found in the 3D Views "Properties" tool shelf overlay. Here "Lines:", "Scale:" and "Subdivisions:" can be used to change the way the Grid appears such that its cells can be altered to account for larger or smaller units of measurement against the default Unit 'types'
When either "Metric" or "Imperial" units are active the "Subdivisions:" property is automatically semi-disabled and greyed-out - it can still be used to further/better define units however
Similarly "Subdivisions:" is semi-disabled when using "Imperial" Units - the setting can be used to modify the predefined defaults based on the "Scene" properties "Scale:" value depending on the unit of measurement required (is generally preferred because Imperial use divisions of odd unit counts - a foot being divided by twelve instead of ten for example
Although "Lines:", "Scale:" and "Subdivisions:" are available universally (notwithstanding the greyed-out caveat mentioned above), they are generally of more use when using Blenders default Units system, "None", largely because it doesn't use any predefined Grid scaling (certainly not in the same way as "Metric" and "Imperial" units).
To change the Grids size in the 3D View left-click the "Scale:" input field and type a new value depending on requirements - the default "Scale: 1.000" equates to one metre, so scaling up "10" would allow models to be build over much larger distances, "0.25" much smaller. Press Enter or click elsewhere on screen to confirm.
Design note: when larger scales are used, although they facilitate being able to build over greater distances, the increased scale also has a similar affect on Grid snapping when manipulating objects, making it somewhat clumsy where smaller details and objects need to be manipulated - it's often preferable to build (details) at normal scale and increase afterwards.
Using "Scale:" each Grid cell is increased in size but not dimensions, in other words each cell shown above is 1 x 1 x scale 2 and not 2 x 2 - the dimensions of the cell don't change, similar in fact to the way the default cube can be rescaled but it's root dimensions remain the same (warranting the use of "Apply" to set the object data to its new sizing)
To change the number of divisions each Grid cell contains amend the "Subdivisions:" value as needed - the default is "Subdivisions: 10", which can be interpreted as ten individual units, or using the Grid to represent 'metres' without setting "Metres" in "Scene" properties, ten blocks of ten centimetres per each larger Grid and so on, changing the value alters the number of divisions and thus the distance each might then represent.
Design note: as mentioned briefly above, when using "Metric" or "Imperial" units of measurement "Subdivisions:" is greyed out, but not disabled. This allows it to be used to override any default or predefined Grid divisions as might be used by the system based on "Scene" properties and "Scale:" found there.
In changing "Subdivisions:" it's possible to set the Grid up so its sub-cells change in relative number and size. This can be used to defined different levels of snap, or units of measurement - if the default "Scale:" is set to "1.000" unit, each subdivision represents 1/10th of that (or blocks of ten centimetre if the base-unit is 1 metre)
To change the size or area the Grid occupies in the 3D View edit the number of "Lines:" setting - the default is "Lines: 16" which shows the Grid occupying a certain percentage of screen real estate; increasing or decreasing "Lines:" changes this so measurement can be assessed relative to the Grid area and respective object size over greater or lesser extents.
Design note: the "Lines:" property only affects the 3D View when in Perspective mode, in Orthographic, changes do not outwardly make any difference (the gird is infinite in Orthographic view).
Using "Lines:" the grid can be expanding or reduced in size relative to the number of grid cells are seen. The default value of "16" allows for quite localised objects to be built of about 160 units before the grid would need to be expanded further, setting a new limit
Separate Units ^
Measurement in Blender is decimal in character and calculated relative to a whatever base unit is being used/has been set. In essence what this means is that by default numerical values always appear as decimal divisions of whole numbers. For example in "Metric" a measurement of "6.5000" is representative of six and a half metres, 'metres' being the base unit, or in "Imperial" six and a half feet, 'feet' being that systems base unit. For clarity Blender has the ability be better define the unit hierarchy so the aforementioned "6.5000" becomes "6m 50cm" under the "Metric" system or "6' 6"" under "Imperial". This is done activating the "Separate Units" property. To do this, in "Units" simply click the checkbox to the left of the sub-heading to activate.
Design note: depending on the actual value, fractions (numbers 'behind' a decimal point) may still appear as decimally place figures relative to using "Separate Units" even when the feature is active, more so for "Imperial" units which would otherwise result in tiny (decimal) fractions, and mean additional math calculations be performed under certain circumstances for accuracy.
In "Imperial" mode units of measurement values are generally relative to the base unit, feet. This results in decimal placements for some values which can be confusing - "6.5" feet is actually six feet six inches not six feet five
With "Separate Units" active, values are split based on their hierarchy; for example in "Imperial" units will be split into "feet" and "inches" (values smaller than the smallest unit may still appear as decimal fractions because Blender cannot correctly display Imperial type fractional increments, i.e. 4/5ths, 7/16ths etc.). Note that "Lines:" and "Scale:" remain unchanged because their values are not expressly units of measurement, and that "Subdivisions:" is greyed-out (but not disabled)
In "Metric", units similarly display as simple decimal fractions by default
With "Separate Units" activated, values are again split into their respective hierarchical relationship, in this instance metres and centimetres. Similarly to "Imperial" properties above note that "Lines:" and "Scale:" remain unchanged with "Subdivisions:" being greyed-out (but still usable)
Valid Units of measurement ^
When selecting either Metric or Imperial Units Blender defaults to using metres or feet as their respective baselines which can be changed up or down depending upon the "Scale:" value set in "Scene" properties - making the value larger would set 'kilometres' or 'miles', smaller and it would be 'inches' and 'centimetres'. In addition to this the unit type is further highlighted by the presence of an appended identifier - "m" for metres, "cm" for centimetres, "'" for feet and """ for inches and so on, or combinations therein depending on the fraction value and whether "Separate Units" is active. In other words valid units of measurement (and scale) are typically indicated as follows (most commonly used);
""" (inches 'quote' ).
"'" (foot 'apostrophe').
- "mi" (miles) .
Design note: values can be represented in more than one way depending on the level of unit separation, i.e. one centimetre can be shown as ("Dimensions:") "X: 1cm", "X: 0.01m" or "X: 10mm", so it's important to make sure the correct descriptor (unit type) is being used to provide proper numerical context. The list above is not exhaustive as other denominators are available but too numerous to mention.
Units in Blender 2.50 to Blender 2.55 ^
Most of the above applies to Blender 2.56 and above, up to and including current versions, the way "Units" are selected from "Scene" properties, the options available, and the way the system works in a general sense are the same from version to version. Prior to this however, for Blender 2.55 and below, significant differences affect the way "Units" works. For example irrespective of the Units system selected (None/Metric/Imperial), "Scale: 1.000" remains fixed, they all use the same distance/base unit. Whilst this may be of no odds for "None" and "Metric" (which use the same base unit), it fundamentally affects the way "Imperial" measurements are calculated because they are then relative to a base unit representing one unit/metre rather than one foot as discussed above.
Design note: as mentioned above, prior to Blender 2.56, when switching "Units" type, i.e. "Metric" and "Imperial" units, the underlying 'scale' value remains unchanged. In other words, each of the measurement sub-systems use the same base unit, which is synonymous with a measurement of one metre. This means Imperial unit calculations must be done relative to metres being the base unit, rather than feet as is the case Blender 2.56 and above; inches require "Scale: 0.0254", feet "Scale: 0.3048", yards "Scale: 0.9144" and so on.
For Blender 2.55 and below, for 'inch' based measurement set "Scale: 0.0254"...
... for 'feet' based measurement set "Scale: 0.3048" (value rounds up for display purposes)...
... and for 'yard' based units set "Scale: 0.9144" (number rounds down for display purposes)
Similarly "Display" properties ("N") differ prior to Blender 2.56 in that "Lines:", "Spacing:" and "Subdivisions:" also behave relative to the base unit being 'metres', which again offers no issue for "None" and "Metric" units of measurement, but does for "Imperial" because the direct translation of the base unit in yards results in a base unit mismatch - "1m" equals "1.094yd". This requires "Spacing:" be edited (select and type corrected value), and "Subdivisions:" be changed to "3", representative of the number of feet in a yard.
Design note: by default in "Imperial" the base unit is "1yd" (one yard) with "Subdivisions: 10". To better approximate the underlying unit (yards) "Subdivisions:" can be set to "3" ("Subdivisions: 3") so the grid snap level is per foot rather than yard or some other non-imperial ratio. This can be done instead of altering "Scale:" in "Scene" properties. Note additionally that switching between Metric and Imperial measurement, the default Scene cube (and other objects) will likely need to be resized or adjusted to better fit the unit type selected (None/Metric/Imperial).
By default in Blender 2.50 through 2.55, when switching between "None" or "Metric", and "Imperial", all measurement data is translated literally so the baseline becomes the Imperial version of a metre, or 1.094 yards (which makes the default cube 2.187 cubic yards). For Grid spacing and object snapping this is incorrect and needs to be adjusted
By editing the "Spacing:" value and setting it to "1yd" the Grid is properly set up with respect to "Imperial" units and the baseline being 'yards'. Additionally "Subdivisions:" can be changed to "3" indicative of there being three feet per yard - note the default Scene cube has been resized to fit the yard base unit (use "Apply" to 'fix' the objects dimensions before continuing)
The net result of this tends to means that when switching, care has to be taken that both the underlying base unit is properly representative of the unit required, and that objects are reset or resized to properly fit the distances involved.
Design note: as mentioned in the previous note changing "Spacing:" is usually enough to set the unit correctly but objects may require some thought to address this problem because distances will need to be converted into the metric/imperial equivalent depending on the direction of unit change (all of which again is relative to the base unit being equivalent to one metre).
When "Scale: 0.3048" is used this sets the grid and base unit to "feet" meaning that each Grid subdivision is then an inch. If the "Spacing:" value is not registering correctly simply change it to "1'" ("Spacing: 1'") and "Subdivisions:" to "12" ("Subdivisions: 12")
When "Scale: 0.9144" is set, representing 'yards', "Spacing:" may again be off and need adjusting to "1yd" ("Spacing: 1yd") along with "Subdivisions: 3" (three feet to one yard)
When both 'metric' and 'imperial' cubes are placed in the same Scene the difference is noticeable. This has obvious effects on modelling because two metres are not the same as two feet, different subdivision levels represent different sub-units etc.
Units in Blender 2.49 or below ^
For Blender 2.49 and below measurement is fixed and non-representative, and although decimal in nature, it's not 'Metric' but a simple global 'unit' based system. In other words distance is measured as "[n] units" and nothing else. Having said this however, it is possible to approximate Metric and Imperial style units using a set of "Grid" options available in "View Properties" - "Spacing:", "Lines:" and "Divisions:", which function in much the same way as described in previous sections above. With these settings the default unit can be taken to represent one foot by changing "Divisions: 10" to "Divisions: 12" (inches per foot), and, either, or, optionally changing "Spacing: 1.00" to "Spacing: 0.0254". For the most part however, leaving the defaults as is should suffice so long as it's remembered what the unit is supposed to represent.
Design note: changing "Grid" properties does not guaranty models made relative to settings used will be compatible with other content made elsewhere. In other words, an 'inch' in Blender 2.49 might not be the same in Blender 2.71 or other applications so some testing may be required. For more information on setting up the Grid in Blender 2.49 and below click here to read "Grid Settings".
In Blender 2.49 and below 'unit' measurement is defined by the use of "Grid" properties - "Spacing:", "Lines:" and "Divisions:". Although older versions of Blender don't have the same dedicated 'units' system, the three aforementioned options can be used to set pseudo-unit properties
Units/Measurement Examples ^
Blender does not (currently) have any pre-set unit selections beyond the primary 'parent' system, i.e. "Metric", "Imperial" or "None" (default). This can make setting up the Scene somewhat confusing because of the differences discussed above. To assist in this the following list contains various property values and settings for the most common sub-units used when making content in Blender;
Design note: the caveat stands that although these may be correct for object size within the Blender environment, comparative testing may be required if exporting materials for use outside of Blender (import basic blocks from target development environment etc. for size comparisons).
Blender 2.56 and above (1 unit = foot).
Inches - (Scene) Scale: 0.083.
Feet - (Scene) Scale: 1.000.
Yards - (Scene) Scale: 3.000.
Millimetres - (Scene) Scale: 0.001.
Centimetres - (Scene) Scale: 0.010.
Metres - (Scene) Scale: 1.000.
Blender 2.55 or below (1 unit = unit/metre).
Inches - (Scene) Scale: 0.0254, (Display) Subdivisions: 4 (quarter inch).
Feet - (Scene) Scale: 0.3048, (Display) Subdivisions: 12 (inches per foot).
Yards - (Scene) Scale: 0.9144, (Display) Subdivisions: 3 (feet per yard).
Millimetres - (Scene) Scale: 0.001, (Display) Subdivisions: 1000.
Centimetres - (Scene) Scale: 0.010, (Display) Subdivisions: 100.
Metres - (Scene) Scale: 1.000, (Display) Subdivisions: 10.
Blender 2.49 or below (1 unit = unit/metre/foot).
Inches - Spacing: 0.0254, Divisions: 4 (quarter inch).
Feet - Spacing: 0.3048, Divisions: 12 (inches per foot).
Yards - Spacing: 0.9144, Divisions: 3 (feet per yard).
Millimetres - Spacing: 0.001, Divisions: 1000.
Centimetres - Spacing: 0.010, Divisions: 100.
Metres - Spacing: 1.000, Divisions: 10.
 Optional. Assumes uniform representation irrespective of unit type.
 High 'Divisions' may be possible but are typically impractical.
As can be seen from the above, Blenders "Units" system, the ability to measure distance in "Metric" or "Imperial" is relatively flexible so long as a few basic principles are followed. Despite this it is important that care be taken if or when switching between systems, there should be a general awareness that any physical discrepancies between objects is more likely down to the differing conditions of each unit system rather than problems with mesh objects themselves.