Picture Frame Cut Calculator: Miter Angles & Lengths

A picture frame cut calculator works out the exact miter angle and the length of each rail (side piece) you need to cut so the frame fits a given artwork or opening. You enter the outside dimensions of the frame and the width of the moulding, and it returns the miter saw angle (usually 45° for a rectangle) and the short-point and long-point lengths for each piece. This page gives you the formulas, a worked example, and the common mistakes that ruin otherwise perfect frames.

Formula

For a rectangular frame, each corner is cut at 45° (assuming the moulding has a flat back and you want a 90° corner joint). The two critical lengths are the long point (the outer edge of the moulding, the longer measurement) and the short point (the inner edge, the shorter measurement).

• Long-point length (horizontal rail) = Frame width − 2 × moulding width
• Short-point length (horizontal rail) = Frame width − 4 × moulding width
• Long-point height (vertical rail) = Frame height − 2 × moulding width
• Short-point height (vertical rail) = Frame height − 4 × moulding width

All dimensions are in inches (or any consistent unit). The miter angle for a perfect rectangle is always 45°—the calculator shows it because many people mistakenly think the angle changes with the frame size. It does not.

Worked Example

Using the default values: frame width = 16 in, frame height = 20 in, moulding width = 2 in.

1. Long-point horizontal length = 16 − 2×2 = 16 − 4 = 12 in
2. Short-point horizontal length = 16 − 4×2 = 16 − 8 = 8 in
3. Long-point vertical length = 20 − 2×2 = 20 − 4 = 16 in
4. Short-point vertical length = 20 − 4×2 = 20 − 8 = 12 in

So you cut two rails at 12 in (long point) and two at 16 in (long point). The short points are 8 in and 12 in respectively—those are the inside edges. The miter angle is 45° on all four corners.

How to Measure and Cut the Rails

Most people measure the frame’s outside dimensions (the total width and height including the moulding). That’s what the calculator uses. When you set your miter saw, you have two choices:

• Cut to the long point: Place the moulding with the outside face up, and measure along the outer edge. This is the standard method—it gives you the exact outside dimension of the frame.
• Cut to the short point: Place the moulding with the inside face up. This is useful when you’re fitting the frame to a specific opening and want the inside dimension to match exactly.

Always mark the moulding on the face that will be visible, and cut with the saw blade on the waste side of the line. For a 45° miter, set your saw to 45° and make sure the moulding is held firmly against the fence—any gap will produce a sloppy joint.

Common Mistakes and How to Avoid Them

The most frequent error is mixing up the long and short points. If you cut to the short point when you meant the long point, your frame will be too small by twice the moulding width. Another classic mistake is forgetting that the miter angle is always 45° for a rectangle—people adjust the angle thinking the size matters. It doesn’t.

Unit mix-ups also happen: if you measure the frame in inches but the moulding in centimeters, the formula breaks. Keep everything in the same unit. Finally, be aware that the formula assumes the moulding has a flat back and that the corners meet at exactly 90°. If your moulding is warped or your saw is not calibrated, the math won’t save you—check the squareness of your cuts with a try square.

When to Use This Calculator vs. a Miter Angle Calculator

This calculator is for rectangular frames only. If you’re building a hexagonal, octagonal, or other multi-sided frame, you need a different formula: the miter angle for an n-sided polygon is 180°/n (for a regular polygon). For example, a hexagon (6 sides) needs 30° miters, an octagon (8 sides) needs 22.5°. This calculator won’t help there—it’s specifically for the most common case: a rectangle.

If your frame is not rectangular but has sides of different lengths, you’ll need a compound miter calculator that accounts for the slope of the moulding. That’s a different tool entirely.

Cutting List and Assembly Tips

Once you have the four rail lengths, write them on the back of each piece with a pencil. Cut the miters, then dry-fit the frame without glue to check that all corners meet flush. If any gap appears, adjust the saw angle slightly (often 44.5° or 45.5° works better on a particular saw) and recut. When you’re satisfied, apply wood glue to the miters and clamp the frame with band clamps or corner clamps. Let the glue dry for at least 30 minutes before handling.

For frames that will hold heavy artwork, add corner splines or a backing board for extra strength. For lightweight frames, glue alone is usually sufficient.

Frequently Asked Questions

Do I need to add a little extra length for the saw kerf?

No. The kerf (the width of the saw blade) is automatically accounted for when you cut to a marked line—you cut on the waste side of the line. The length you measure is the finished length of the piece, not the length of the board you start with. So you don’t add anything for the kerf; you just need to start with a board longer than the longest rail.

What if my frame is not a perfect rectangle?

If the sides are not parallel or the angles are not 90°, the 45° miter won’t fit. You need to measure each corner angle with a protractor or a bevel gauge, then divide by two to get the miter cut for that corner. For example, if a corner is 88°, you cut each side at 44°. That’s a custom angle, not something this calculator handles.

Can I use this calculator for a frame with a mat?

Yes, but you need to decide whether you’re measuring the outside of the frame or the inside of the mat. If you want the frame to fit around a mat, measure the outside of the mat (which is the inside of the frame) and then add the moulding width twice to get the outside frame dimensions. Then enter those outside dimensions into the calculator.

Why is the miter angle always 45°?

Because a rectangle has four 90° corners. When you cut two pieces at 45° and join them, they form a 90° corner. The size of the frame doesn’t change the angle—only the shape does. If the frame were a triangle, the miter would be 60°; a pentagon, 36°; and so on.

What does “long point” and “short point” mean?

When you cut a miter, the cut is at an angle, so the piece has two different lengths: the outer edge (long point) and the inner edge (short point). The long point is the measurement you use when you want the frame’s outside dimensions to match your input. The short point is the measurement of the inside edge, which is useful for fitting a specific artwork or glass size.

Can I cut the miters with a hand saw?

Yes, but you’ll need a miter box to guide the saw at a precise 45° angle. Hand saws are slower and less accurate than power miter saws, but with a good miter box and a sharp saw, you can get acceptable results. Just be sure to clamp the moulding securely and cut slowly.

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in w | Frame width (outside) | in | 16 | 0 | 200
in h | Frame height (outside) | in | 20 | 0 | 200
in m | Moulding width | in | 2 | 0.1 | 12
out ang | Miter angle | ° | 45 | 1
out ls | Long-point length | in | w - 2*m | 2
out ss | Short-point length | in | w - 2*m - 2*m | 2
out lh | Long-point height | in | h - 2*m | 2
out sh | Short-point height | in | h - 2*m - 2*m | 2