How to use french cleat cut geometry
What this calculation does
This geometry estimate splits a measured board through its center at 45 degrees and projects the blade kerf along that cut.
Measure board width and actual thickness, then enter kerf measured normal to the blade.
Enter the measurements
- Board width across the split: Measure board width across the split.
- Actual board thickness: Measure actual stock thickness.
- Measured blade kerf normal to blade: Use blade kerf measured normal to the blade.
Formula and variables
Projected kerf = blade kerf ÷ cos(45°). Short face = board width ÷ 2 − thickness ÷ 2 − projected kerf ÷ 2. Long face = board width ÷ 2 + thickness ÷ 2 − projected kerf ÷ 2.
Projected kerf divides normal kerf by cos 45 degrees. Each face starts from half board width, offsets by half thickness and loses half the projected kerf.
Read the result
The 6×0.75 in board with 0.125 in normal kerf gives about 2.537 in and 3.287 in faces.
Worked example
Inputs: 6 in board, 0.75 in thickness and 0.125 in blade kerf
- Projected kerf = 0.125÷cos45° ≈ 0.1768 in.
- Faces are 3−0.375−0.0884 ≈ 2.5366 in and 3+0.375−0.0884 ≈ 3.2866 in.
Expected result: 0.1768 in projected kerf; 2.5366 in short face and 3.2866 in long face
Frequently asked questions
Does this rate a wall cleat for a load?
No. It calculates cut geometry and does not assess fasteners, wall type or supported load.
Is kerf measured along the angled face?
The input is normal to the blade; the formula projects it using the stated 45 degree cut.
Conditions, presets and technical notes
Assumes a centered 45° through-cut and the stated kerf convention. This calculates cut geometry only; it does not determine fasteners, wall type, cleat engagement or supported load.