Miter Angle Calculator
Miter angle
45.00°
How it works
A miter (or mitre) is an angled cut across the width of a board, used to create frames, corners, and angled joints. Getting the miter angle wrong wastes expensive material and requires recuting. The Miter Angle Calculator computes miter angles for frames, polygons, angled corners, and compound miters (cuts angled both horizontally and vertically).
**Simple miter for a frame corner** For a rectangular 90° corner: each piece is cut at 45° (miter = 90° / 2 = 45°). The two 45° cuts meet to form the 90° corner. General formula: miter angle = corner_angle / 2.
**Polygon frames** For a regular polygon with N sides: each interior corner = 180° × (N−2) / N. Miter angle = 90° − (180°/N). Pentagon (5 sides): interior angle = 108°; miter = 54°. Hexagon: interior = 120°; miter = 60°. Octagon: interior = 135°; miter = 67.5°.
**Compound miter (crown moulding)** Crown moulding is cut at a compound angle — both the miter (horizontal angle) and bevel (vertical angle). Crown moulding sits at a specific spring angle (typically 38° or 45°) relative to the ceiling/wall. Given the spring angle and corner angle, the compound miter formula is: miter_angle = arctan(sin(corner/2) / tan(spring_angle)). Compound miters require both the saw's miter scale and bevel scale to be set correctly.
**Common angle reference** Corner angle → miter saw setting (for outside corners): 90° corner = 45°. 135° corner (obtuse) = 22.5°. 60° corner (acute) = 30°. 45° corner = 22.5°. The calculator shows both the inside and outside corner miter settings.
Privacy: all calculations run in the browser. No data is transmitted.
Frequently Asked Questions
- They do — mathematically. If both pieces are cut at exactly 45° and joined, the corner is exactly 90°. The problem is real-world accuracy: a 0.5° error on each cut (both cut at 44.5° instead of 45°) produces a gap. Additionally, the surfaces being joined must be flat — any twist or warp in the wood prevents tight contact. Tips for perfect miters: use a sharp, fine-toothed blade; support the workpiece fully; test with scrap pieces first; use a reliable stop/fence system. For painted trim, small gaps can be filled with caulk — for stained hardwood, a test cut to check the actual corner angle produced by your saw is essential.
- Crown moulding is a compound miter — both the miter angle and bevel angle of your saw must be set correctly. The settings depend on the moulding's spring angle (typically 38° or 45°). For a 38° spring angle moulding at a 90° inside corner: miter angle = 31.62°, bevel angle = 33.86°. For a 45° spring angle at 90° inside corner: miter angle = 35.26°, bevel angle = 30°. An alternative technique: cope the inside corner — one piece runs straight to the wall; the second piece is coped (back-cut along the profile) to overlap the first. Coped joints stay tight as wood expands/contracts with humidity; mitered inside corners tend to open.
- A standard miter saw cuts at a horizontal angle (miter) but the blade is vertical. A compound miter saw also tilts the blade (bevel cut). A sliding compound miter saw extends the blade on rails for wider boards. You need a compound miter saw for: crown moulding (requires simultaneous miter and bevel); angled fascia boards on roof rafters; any cut requiring both a horizontal and vertical angle simultaneously. For simple 90° corner picture frames, picture rail, or standard door casing: a standard miter saw with only a miter adjustment is sufficient. Compound miters can also be cut by laying the workpiece flat and cutting only a miter (the 'flat cut' method for crown), if you know the flat-cut angles.
- A regular hexagon has 6 sides and 6 interior angles of 120° each. For each corner of the frame: miter angle = corner_angle / 2 = 120° / 2 = 60°. Set your miter saw to 60° (or equivalently, 30° from the fence on most saw scales where 0° = square). Cut both ends of each of the 6 pieces at 60°. Dry-fit before gluing — hexagonal frames are unforgiving of small angle errors because errors accumulate across 6 joints. Use a strap clamp (band clamp) to apply even pressure at all joints simultaneously while glue sets.