Picture Frame Mat Calculator

Use the mat board size, not the outside frame size

The nominal frame size often refers to what fits inside, while the outside molding is larger. Measure your actual outer mat before marking a cut. Decide which artwork edges should be hidden. If a photograph has a printed white margin that you want concealed, enter the image dimensions that define the intended coverage and verify that the full paper still fits behind the board.

Window and weighted-border formula

For artwork width A and height B with per-edge overlap o, opening width = A - 2o and opening height = B - 2o. For outer mat width W and height H, each side border = (W - opening width) / 2. If bottom-minus-top difference is d, top = (H - opening height - d) / 2 and bottom = top + d. The opening center moves upward by d / 2.

Remaining mat face area = W × H - opening width × opening height, in square inches. This is face geometry, not the area of board to buy: the full outer rectangle is still required. Artwork larger than the mat, overlap that erases the opening, and weighting that makes the top border negative are rejected visibly.

Worked example: a 10 × 14 image in a 16 × 20 mat

With 0.125 inch overlap on each edge, the opening is 9.75 × 13.75 inches. Side borders are 3.125 inches each. With the bottom 0.5 inch deeper than the top, top border is 2.875 inches and bottom is 3.375 inches. The center is 0.25 inch above the mat center. Mat face outside the opening is 185.9375 square inches.

Transfer the result to a cutting layout

Mark the window edges at the reported distances from the board edges, keeping top and bottom clearly labeled. Confirm that the two side borders plus opening width equal outer width, and that top plus opening height plus bottom equal outer height. A bottom-weighted layout is not vertically centered: rotating the board reverses its effect. Try a paper mock-up before cutting expensive board.

Assumptions and practical limitations

This is exact rectangular geometry with user-chosen overlap and visual weighting; no external standard or empirical recommendation is needed. It assumes a square board, a straight-sided image, one opening, and no rotation. It does not account for bevel depth, blade offsets, frame lip coverage, float mounting, multiple windows, or double-mat reveals. Follow your cutter's instructions for which face and line to cut. Overlap hides an edge but does not replace appropriate mounting or archival advice.

Inputs and outputs are inches, with up to four displayed decimals. Convert decimals to your ruler's subdivisions conservatively and remeasure; dimensional rounding can change edge coverage. A mathematical zero border is allowed, but it does not imply the board has sufficient strength or a usable cutting margin.

Related tools

• Pottery shrinkage calculator for planning a ceramic tile's final dimensions before framing.
• Centimeters to inches for translating metric artwork measurements.

Last updated: 2026-10-09. Verify cut lines on a mock-up before cutting.