Mat Border Calculator
Find the outer mat size and frame opening from your artwork size and border width — or work backward from a fixed frame to a border that comes out even on every side.
Result
The border is applied evenly on all four sides to get the outer mat size, which is also the size the frame’s inner rabbet needs to clear.
Why the border width drives everything downstream
A mat's border width is one of the first decisions in framing a piece, because it sets the outer mat size, which in turn sets the frame's inner rabbet opening. Get the border wrong and everything built on top of it — the frame order, the glazing size, the backing board — is wrong too. A wider border (2.5–4 inches is common for gallery-style framing) gives artwork visual breathing room; a narrower border keeps the finished piece more compact.
When you're working from a frame you already own rather than choosing a border from scratch, the reverse calculation tells you exactly what border that frame gives you — and whether it lands even on all sides or needs the artwork's own crop adjusted first.
Frequently Asked Questions
How is the outer mat size calculated?
The border width you choose is added twice to each artwork dimension — once for each side — so an 8×10 piece with a 2-inch border becomes a 12×14 outer mat (8 + 2 + 2, and 10 + 2 + 2). That outer size is also what the frame's inner rabbet needs to clear.
What's the difference between the mat window and the outer mat size?
The window is the opening cut into the mat that shows your artwork; the outer size is the full mat board, border included. By default the window matches your artwork size exactly. If you enter a mounting overlap, the window is cut slightly smaller than the artwork on each side, so the mat itself overlaps and holds the artwork's raw edge in place — a common practice for unmounted prints and photos.
How does the reverse mode work — solving for an even border?
Given a fixed frame opening and your artwork size, the calculator subtracts the artwork dimensions from the frame opening and splits the difference by two on each axis: border = (frame opening − artwork size) ÷ 2. If the resulting left/right border doesn't match the top/bottom border, the result is flagged as uneven — a sign the frame opening isn't proportioned to give a symmetrical mat around that artwork.
Should the border be the same width on all four sides?
A uniform border on all sides is the standard, classic look. Some framers add slightly more width at the bottom (a 'weighted' or 'bottom-heavy' border) so the mat doesn't look like it's sliding down under its own visual weight — this calculator computes a uniform border; add extra at the bottom yourself if you want that effect.
Do I need to account for the mat overlapping the artwork edge?
Only if you're not mounting the artwork some other way (like hinging it to a backing board). If the mat itself is what holds a print or photo in place under the window, a small overlap of about 3mm (roughly 1/8 inch) on each edge is typical. If the artwork is fully mounted or hinged, you can leave the overlap at 0 and let the window match the artwork size exactly.
Educational estimate — not professional conservation-framing advice; for valuable or fragile artwork, consult a professional framer before cutting mats or mounting.