Flip Normals (inverted faces)

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In newer versions of Blender the previously available Tool Shelf buttons Flip Direction and Recalculate, typically used to flip inverted surfaces that made meshes appear inside-out, have been moved to the main Mesh menu upper-left of the 3D View. As the option is now context sensitive it only appears as a menu option in Edit Mode.

Design note: the Mesh menu is specific to Edit Mode and does not appear unless that particular editing context is active.

Flip Selected Faces

To access the option and/or correct an inside-out or inverted face, first select the errant mesh element in Edit Mode [1] (using vertex, edge or face) and from the Mesh menu upper-left click Normals [2] then Flip [3] from the options that appear – Mesh » Normals » Flip. Alternatively use Alt + N to access the Normals context menu then select Flip. The highlighted inverted face/s will immediately invert based on their current orientation.

Design note: when selecting errant faces using vertices or edges make sure all component elements are included to the degree that the inverted faces highlight.

Meshes or faces that appear inside out or inverted are fixed from the 3D Views 'Mesh' menu
The option to flip or invert faces [1] is an Edit Mode option only (accessible only when editing meshes), click Mesh » Normals » Flip [2 & 3] with an individual face or group selected.

Recalculate Faces

As an alternative to selectively inverting faces, or re-orientating a large mixed selection so everything points in one direction,  inwards or outwards, use Recalculate Outside (Shift + N) or Recalculate Inside (Shift + Ctrl + N) options. To do this and force the orientation in one direction or the other, in Edit Mode select the entire mesh (A) and access the Mesh menu. Here select Normals » Recalculate Outside to flip faces/selections outwards, or Normals » Recalculate Inside to invert selections inwards – Mesh » Normals » Recalculate Outside/Inside.

Design note: in previous versions of Blender Recalculate tended to orientate faces randomly depending on the predominance of inwards or outwards facing surfaces prior to use. For Blender 2.8+ this is no longer the case, Recalculate Inwards and Recalculate Outwards forceably re-orientate selections based on the option selected, either in or out but not both.

IF faces are randomly inverted use Recalculate to flip everything in one direction
For instances where surfaces are randomly facing one way or the other [4], or there are too many to select and manually process, use Recalculate Inwards/Outwards [5] to force everything to face the same direction, in or out, regardless – make a selection (individual, group or all) then click the Mesh menu, Normals then Recalculate Inwards/Outwards.

Face Orientation

To assist determining which way a face might be pointing, especially if Backface Culling is not disabled in Viewport Overlays or Viewport Shading options (depending on the viewport display mode) or a given materials Settings properties, Blender 2.8+ includes an option to colour tint faces based on their orientation, RED for surfaces pointing inwards and BLUE for those pointing outwards, making it much easier to see at a glance what’s going on, which then allows for individual faces to be flipped or recalculated as needed. To use, click the Overlays button upper-right of the 3D View [6] and from the menu select Face Orientation [7] – Overlays » Face Orientation. Everything will immediately tint blue/red based on their orientation. To disable, repeat, select Overlays » Face Orientation.

Design note: if X-Ray (Alt + Z) is active for any of the Viewport Shading modes, Wireframe, Solid etc., Face Orientation won’t work.

Backface Culling is off
Face Orientation used to determined which way faces point
If Backface Culling is disabled (off) it can be difficult to determine which way a given surfaces is pointing because both front and back of the mesh are the same. In this situation using the Overlays [6] setting Face Orientation [7] can help as it tints surfaces based on their orientation, inwards or outwards.

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