Miter Angle Calculator

This miter angle calculator will help you find the miter angles needed to make a tight miter joint for your next project.

Miter angle30°180° ÷ 6 — cut both ends of every piece at this

Mitre saw setting30°measured from square, which is how saws are marked

Interior angle120°the angle at each corner of the finished frame

Exterior angle60°

Sides6hexagon

Total of all miters360°two cuts per joint, n joints — always 360°

Perimeter72 in6 ft

Across the flats20.7846 ininside dimension between opposite sides

Across the corners24 in

Area374.123 sq in

The formula

miter = 180° ÷ n for an n-sided frame; bevel from the spring angle

Halve the corner

Two pieces meeting at a corner each take half of it, so a 90° corner needs two 45° cuts. For a regular polygon the corner angle is 180° − 360°/n, so each miter is 180°/n: 45° for a square, 30° for a hexagon, 22.5° for an octagon.

Compound miters are harder because the moulding leans off the wall. The cut then has to be right in two planes at once, which is why a crown moulding in an ordinary square corner needs 31.6° of miter and 33.9° of bevel rather than a simple 45°.

Whatever the arithmetic says, measure the actual corner. Very few walls are truly square, and a 2° error at the wall becomes a visible gap at the miter.