What this calculator does
A mitre cut joins two pieces of trim, skirting, architrave or framing at a corner without either piece running square into the other. The rule behind every mitre cut is the same: each piece is cut at half the corner angle it needs to form, so the two angled faces meet flush when the pieces come together.
For the standard 90 degree corner found on most doorways, windows and box frames, that means two 45 degree cuts, one on each piece. Odd corners, bay windows and multi-sided frames need a different split, and that is where a mitre cut calculator saves the trial and error, since halving an angle that is not 90 by eye is easy to get wrong.
The formula
Divide the corner angle you want to form by two. That figure is the mitre angle cut into each of the two pieces, measured as the saw is normally set, from a square (0°) crosscut. The two cuts, one on each piece, always add back up to the original corner angle when the joint is closed.
| Term | Meaning |
|---|---|
| Corner angle | The angle of the finished corner where the two pieces meet, measured on the inside or outside of the joint, whichever face you are working to. |
| Mitre angle | The angle each piece is cut at, equal to half the corner angle, set on the saw as a rotation away from a straight, square cut. |
| Angle from the edge | The same cut expressed as an angle measured from the long edge of the board rather than from square, which is 90° minus the mitre angle. |
The inputs explained
| Field | What to enter |
|---|---|
| Corner angle to form (°) | The angle of the corner the two pieces need to form once joined. Leave it at 90° for a standard square corner, or change it for an odd-angled wall, bay window or multi-sided frame. |
When to use it
Fitting skirting or architrave around a square corner
The everyday case: an internal or external 90° corner needs two 45° mitre cuts, one on each length, so the profile continues around the corner without a visible butt joint.
Framing a bay window or splayed wall
Walls that meet at something other than 90°, such as a 135° bay window corner, need a different mitre split; halving the actual corner angle rather than assuming 45° avoids a gap in the joint.
Building a multi-sided frame
A frame with more than four sides, such as a hexagonal mirror surround, has a corner angle set by the shape itself; the mitre saw setting for each piece follows directly from that corner angle rather than a fixed 45°.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
Mitre angle for common corner angles
The mitre angle needed on each piece for a range of corner angles, from a shallow splay to a wide obtuse corner.
| Corner angle | Mitre angle per piece (saw setting from square) | Angle measured from the board edge |
|---|---|---|
| 30° | 15.0° | 75.0° |
| 45° | 22.5° | 67.5° |
| 60° | 30.0° | 60.0° |
| 90° | 45.0° | 45.0° |
| 120° | 60.0° | 30.0° |
| 135° | 67.5° | 22.5° |
| 150° | 75.0° | 15.0° |
Questions
What angle do I cut for a 90 degree corner?
Each piece is cut at 45°, half of the 90° corner angle, so the two cut faces meet flush when joined.
Is the mitre saw angle the same as the corner angle?
No. The mitre saw angle set on each piece is half the corner angle being formed, since two of those cuts join together to make the full corner.
Does this work for outside corners as well as inside corners?
Yes. The corner angle is whichever angle the two pieces need to form where they meet; the halving rule is the same whether that corner is internal or external, only the piece is flipped to present the correct face.
What if the two walls do not meet at a clean angle?
Measure the actual corner angle with an angle finder or protractor rather than assuming 90°, and enter that measured value; an inaccurate corner angle is the most common reason a mitre joint ends up with a visible gap.
For the roof-pitch side of a build, see the pitch calculator. For drainage fall rather than a joinery angle, see the pipe fall calculator.