Mat Board Calculator — Dimensions & Opening Size

A weighted bottom border is slightly wider than the top and sides, typically 6-19 mm (1/4 to 3/4 inch) extra, with 10 mm a common default and at least 13 mm (1/2 inch) recommended to make the weighting clearly visible. This compensates for an optical illusion that makes equally bordered work appear to sag toward the bottom. It is standard practice in professional framing.

Standard mat borders are 55-75 mm (2.2-3 inches). For small works under A4, 55 mm works well. For larger pieces, 65-75 mm gives better visual balance. Very large works may use 100+ mm borders.

The mat opening is cut slightly smaller than the artwork — about 1/8 to 1/4 inch (3-6 mm) on each side, 3 mm by default here — so the mat edges hold the artwork in place and no gaps or mount board show through. You can change the overlap under Advanced options. This overlap is standard in professional framing.