Picture Mat & Frame Size Calculator

How it works

window = artwork − 2 × overlap · outer width = window + 2 × border; outer height = window + top + bottom (bottom = border × (1 + weighting)) · frame mode: side border = (frame − window) ÷ 2; top = (frame height − window) ÷ (2 + weighting)

A mat does three jobs — it holds the artwork flat and off the glass, hides its edges, and gives the eye a resting margin — and its arithmetic runs in two directions. Starting from the art, an overlap of a few millimetres per edge sets the window, a chosen border sets the outer size and therefore the frame, and a slightly heavier bottom border corrects the optical illusion that equal borders make a picture sit low. Starting from a frame you already own, the same relations give the borders that result, which is how you judge whether a print will look cramped or lost. Everything else — glass, backing, a second mat with a small reveal — is derived from the outer dimension with a couple of millimetres of clearance in the rebate.

Worked example

An A4 print (21 × 29.7 cm) with a 4 mm overlap has a 20.2 × 28.9 cm window. With 6 cm borders and a 15% heavier bottom (6.9 cm) the mat and frame come to 32.2 × 41.8 cm — so cut the mat to a standard 40 × 50 cm frame instead, which gives 9.9 cm sides and a 9.8 / 11.3 cm top and bottom, a generous gallery proportion. The glass and backing are cut to 39.8 × 49.8 cm.

Frequently asked questions