Occasionally I need to make a floating frame for a canvas painting. This page is for reference and board length calculations. Full instructions with pictures and proccess can be found on Sweet Beast's blog.

These calculations are intended for 1 ¹⁄₂in. deep canvas.

For this type of a frame all the boards (both the "inside"/back and "outside"/frame) should be (true) ¹⁄₂in. thick.

The "outside"/frame boards should be 2 ¹⁄₈in. wide, which will give you ¹⁄₈in. "overhang". (Meaning the frame will extend ¹⁄₈in. over the painting.)
The "inside"/back boards should be at least 2in. wide, but they can be a bit wider.
(They can be ripped at the same time as the outside boards.)

Using the lengths in this calculator will give a gap of ¹⁄₄in. between canvas and painting on all four sides.