Picture Frame Size Calculator

Picture frame sizing and matting involve three distinct measurements: the artwork size, the mat (passepartout) opening and border, and the frame’s rabbet (recess) size. Getting these right ensures the artwork is properly presented without covering important content or leaving unintended gaps.

Frame rabbet size formula: Frame Inner Dimension = Artwork Width (or Height) + 2 × Mat Border Width − 2 × Mat Overlap

Standard mat overlap onto artwork: 0.25 inches per side (to hide the edge of the artwork/paper)

Mat opening (window) size: Mat Window = Artwork Dimension − 2 × Overlap

For a 5″ × 7″ photo with 0.25″ overlap per side: Mat window = (5 − 0.5) × (7 − 0.5) = 4.5″ × 6.5″

Mat border width, and what each one says:

• Narrow, 1.5 to 2 inches: modern, tight, works for a gallery wall of many small pieces
• Standard, 2 to 3 inches: safe for almost anything
• Gallery, 3 to 4 inches and up: makes a small piece feel important, and it is the single cheapest way to make a print look expensive
• Bottom-weighted: cut the bottom border half an inch to an inch wider than the other three. The eye reads perfectly even borders as top-heavy, so a symmetrical mat looks subtly wrong and nobody can say why

Frame glass and backing: Both are cut to the frame opening, not to the artwork. Glass sits flush inside the rabbet lip, and the backing board goes behind everything at the same size.

Multi-opening mats: Count the windows and the border areas, then add 10% for cutting waste. Nobody cuts a five-window mat right the first time.

Archival against standard framing:

• Archival: acid-free mat and backing, UV-filtering glazing, and no adhesive ever touching the artwork itself. Use hinges, not tape
• Standard: chipboard mat and ordinary glass. Fine for a poster or a decorative print, wrong for anything original or for photographs meant to survive

The failure mode with cheap board is specific and slow: acid migrates out of the mat into the paper and leaves a brown line exactly where the window edge sat. By the time you notice, it is permanent. If the piece matters, the acid-free upgrade costs very little at the point of framing and nothing can undo it afterwards.

Common frame sizes (US standard): 4×6, 5×7, 8×10, 8.5×11, 11×14, 12×16, 16×20, 18×24, 20×24, 24×36 inches. Stick to these and you pay shelf prices. Step outside them and a custom frame can cost several times the piece it holds.

Worked example: Original watercolor: 9″ × 12″. You want a 3″ mat border on all sides, with 0.25″ overlap onto the artwork.

• Mat window opening: (9 − 0.5) × (12 − 0.5) = 8.5″ × 11.5″
• Frame inner dimension: (8.5 + 6) × (11.5 + 6) = 14.5″ × 17.5″

Note that this is not 15 × 18. The quarter-inch overlap on each side takes half an inch off both dimensions, and that half inch is exactly the amount that decides whether a ready-made frame fits.

The smallest stock frame that swallows 14.5 × 17.5 is 16 × 20. To fill it properly you widen the mat rather than the artwork: 3.75″ on the left and right, 4.25″ on the top and bottom. Uneven borders like that are normal in custom matting and invisible once hung, and a custom-cut mat costs a fraction of a custom frame.

The thing to buy in the right order: the frame first, then the mat cut to fit it. Doing it the other way round is how people end up paying for a bespoke frame around a mat they could have trimmed for nothing.