Miter Angle Calculator
Find the exact miter saw setting to join trim at any corner or around an equal-sided frame.
Reviewed by the ToolNestr Editorial Team — July 2026
How the miter angle calculator works
A mitered joint hides the end grain of trim by splitting a corner between two pieces. Because the two cuts must together turn through the whole corner, each piece is cut to exactly half of the turn. If you measure the angle where the walls actually meet, the miter on each piece is (180 − corner angle) ÷ 2 — a 90° corner gives 45°, a wider 135° corner gives 22.5°.
For a closed frame with equal sides — a picture frame, an octagon window casing, a planter box — there is an even simpler rule. Each miter equals 180 divided by the number of sides. Four sides give 45°, six give 30°, eight give 22.5°. This works because the exterior turns of a closed polygon always add up to 360°, shared equally across its joints, and each joint splits its turn between two pieces.
Most miter saws read zero when the blade is square to the fence, so the number you dial in is the miter measured from 90°, which is exactly the value this calculator gives. The same setting works for inside and outside corners; only which face of the trim ends up longer changes. Real corners are rarely perfectly square, so measuring the actual corner angle beats assuming 90° when a tight joint matters.
Miter geometry and joint layout conventions follow guidance published by the Architectural Woodwork Institute (AWI) Quality Standards for finish trim and casework.
Measure or count
The real corner angle, or the number of sides.
Read the miter
The angle each piece is cut to.
Set the saw
Dial that value from the square (90°) position.
The formula explained
Corner-angle mode
miter = (180 − corner) ÷ 2. A 90° corner: (180 − 90) ÷ 2 = 45° on each piece.
Frame mode
miter = 180 ÷ N. An octagon (8 sides): 180 ÷ 8 = 22.5°.
Saw setting
The saw reads the miter from square, so the displayed miter is the number to dial in directly.
Worked example
Baseboard around an out-of-square corner measured at 96°.
Tips for a tight miter joint
Measure, don't assume
A cheap angle finder or a simple bevel gauge held against both walls takes thirty seconds and often reveals a corner is off by two or three degrees. That small difference is the whole reason a "45° everywhere" approach produces visible gaps on real trim work.
Cut a scrap test piece first
Dial in the calculated miter, cut two short offcuts, and dry-fit them before touching the actual trim. It costs a few inches of scrap and confirms the saw's angle indicator is accurate — miter saw scales can drift out of calibration over years of use.
Real-world context
Picture-frame and cabinet-door work is where the frame-mode formula gets used most. A five-sided arched window casing needs 180 ÷ 5 = 36° per miter, while a hexagonal planter box comes out to 180 ÷ 6 = 30° per miter — both far from the 45° most people assume every miter should be, which is exactly why counting sides first matters before setting the saw.
Real rooms rarely give you an honest 90°. A bay window nook measured with a protractor at 112° needs (180 − 112) ÷ 2 = 34° per piece, noticeably shallower than the default 45° a carpenter might dial in out of habit. Skipping the measurement step on an out-of-square wall is one of the most common causes of a visible gap at the joint.
Older homes settle unevenly, and closet or hallway corners often drift a few degrees from square. A corner measured at 87° needs (180 − 87) ÷ 2 = 46.5° per piece — just over a degree past 45°, but enough that an unmeasured 45° cut would leave a visible sliver of a gap once both pieces are nailed up.
Multi-sided frames come up often in built-in cabinetry and window casings too, not just picture frames. A ten-sided decorative surround, for instance, needs 180 ÷ 10 = 18° per miter — the frame-mode formula scales cleanly to any number of equal sides, which is why it is worth confirming the side count before cutting rather than guessing from a rough sketch.
Common misconceptions
"Every inside corner is 90°, so every miter is 45°." Only on paper. Framing, drywall, and settling routinely push real corners a few degrees off square. Measuring the actual corner and halving it — rather than assuming 45° — is what closes the gap on a finished joint.
"The miter angle changes between an inside and an outside corner." The angle is identical. What differs is which face of the trim — the back or the front — ends up longer once the piece is cut and installed. The saw setting itself comes from the same (180 − corner) ÷ 2 formula either way.
Related calculators
Frequently asked questions
How do I calculate a miter angle for a corner?
Measure the corner angle where the two walls meet, then set each piece to a miter of (180 − corner) ÷ 2. A square 90° corner needs a 45° miter on each piece; a 135° corner needs 22.5°.
What miter angle do I cut for a picture frame?
A four-sided frame is cut at 45° on every joint. For an equal-sided frame with any number of sides, each miter is 180 ÷ N degrees — 30° for a hexagon (6 sides), 22.5° for an octagon (8 sides).
Why do the two pieces split the angle?
The joint between two pieces of trim must add up to the full corner. When both pieces are cut to the same angle, each one carries half the turn, so the cuts meet cleanly in the middle of the corner without a gap.
Does an inside corner differ from an outside corner?
The miter angle is the same for a given corner angle — what changes is which face of the trim is longer. For an inside corner the back of the trim is longer; for an outside corner the front is longer. The saw angle setting itself does not change.
Sources & references
This tool uses standard formulas and reference values from:
- • American Concrete Institute — ACI 318, Building Code Requirements for Structural Concrete. concrete.org
- • ICC — International Residential Code (IRC), span, footing and framing tables. codes.iccsafe.org
- • APA – The Engineered Wood Association, allowable span and load guidance.
Estimates for planning only. Span, load and code values vary by jurisdiction — verify against your local adopted code and a licensed engineer before building.