Miter Angle Calculator
The miter saw setting is not your corner angle. For a 60 degree corner you set 60 degrees off the stop, not 60 on the base. This calculator gives you both numbers and the true length of the mitered piece.
Results
Enter your measurements above and press Calculate.
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How this is calculated
The two numbers a miter saw needs are not the same number, and that is where most frames go wrong.
- Miter setting = (180 - corner angle) / 2, the angle set on the saw table
- Bevel = corner angle / 2, the blade tilt used for a compound cut such as crown moulding
- True hypotenuse = nominal length / cos(corner angle / 2), the long point to long point length the piece actually needs
- End trim loss = nominal length x (cos - sin) of the half angle, the length consumed by the two angled ends
A 90 degree corner gives 45 degrees on each cut, the case everyone knows. A 120 degree outside corner gives 30 degrees, and that is where a square-cut habit leaves a visible gap. Miter and bevel always sum to 90 degrees, so knowing one lets you check the other. The hypotenuse and trim figures assume the nominal length is measured across the inside face of the frame, which is how mat and frame suppliers quote it.
Questions people ask
What miter angle for a 90 degree corner?
A 90 degree corner gives a 45 degree miter on both pieces. Each piece is set 45 degrees off the fence on a miter saw.
How do I miter a 120 degree angle?
Set the miter gauge to 30 degrees — that is (180 − 120) ÷ 2. A two axis miter saw sets 30 degrees on the miter and 30 degrees on the bevel.
Should I measure from the inside or outside edge?
Measure the nominal width across the back of the molding, and cut the front face slightly long. The face reference is always the widest visible dimension.
Why does my miter not close on the backside?
It means the angle is off, or the stock is not square, or the blade is not sharp. Mitered joints are unforgiving — a 0.5 degree error leaves a visible gap at 24 inches.
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