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About this tool
Miter saw angle, long and short point lengths and total moulding for rectangular or many-sided picture frames.
A picture frame miter calculator gives the saw angle and the exact cut lengths for a closed frame of any number of sides. The angle comes from the fact that a frame turns through 360° in total, so each corner turns 360/n and each end is cut 180/n away from square — 45° for a square, 30° for a hexagon, 22.5° for an octagon. Lengths follow from the geometry of a regular polygon: the outside of each rail is longer than the inside by 2 × moulding width × tan(180/n), which for a rectangle is the familiar rule of opening plus twice the moulding.
Open Picture Frame Miter Calculator on AltFTool — it loads instantly in your browser.
Enter the values you already know.
Fine-tune the options to match your scenario.
Read the result and use it in your planning or reporting.
Cutting lists go wrong when only one is given — both ends of every rail are listed.
Triangles, pentagons, hexagons, octagons and more, with the same geometry as the rectangle case.
Blade kerf is taken off at every cut and the result is turned into whole sticks of moulding.
45° on a four-sided frame, because each of the eight ends must take half of the 90° corner. The general rule is 180 ÷ the number of sides, so a hexagon is cut at 30°, an octagon at 22.5° and an equilateral triangle at 60°.
For a rectangle, each rail's long point equals the rabbet dimension plus twice the moulding width, so the total is 2 × (width + height) + 8 × moulding width. A 300 × 400 mm opening in 40 mm moulding needs 2 × 700 + 320 = 1720 mm before kerf and waste.
The long point is the outside edge of the rail and the short point is the inside edge at the rabbet; they differ by 2 × moulding width × tan(180/n) per rail. Frame shops normally measure to the long point, so always check which one a cutting list means before you cut.
About 3 mm (1/8") larger in total across each dimension so the artwork and mat drop in without binding, and the glass is usually cut around 1.5 mm under the rabbet size for the same reason. Paper expands with humidity, so err towards the larger clearance for unmounted works on paper.
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