A mat, or passe-partout, is the board with a beveled window that sits between a picture and the glass. It keeps the artwork away from the glass, gives it breathing room and makes a small print look important. Getting the window and borders right is simple arithmetic, and this calculator does it in either inches or centimeters.
The formulas
- Window = artwork size − 2 × overlap
- Side borders = (frame width − window width) ÷ 2
- Top border = (frame height − window height − bottom weighting) ÷ 2
- Bottom border = top border + bottom weighting
The overlap is how much the mat covers each edge of the artwork so it does not fall through the window: usually 1/8 to 1/4 in (3 to 6 mm). For a photo with a white margin you might let the window show a little of it; for a deckled watercolor you can float it, with a negative overlap that leaves a gap of paper around the art.
Example
An 11 × 14 in print in a 16 × 20 in frame with a 1/4 in overlap gets a 10.5 × 13.5 in window. The sides are 2.75 in. With half an inch of bottom weighting, the top border is 3 in and the bottom 3.5 in.
Why weight the bottom?
A window centered exactly looks slightly low to the eye. Framers traditionally make the bottom border a little wider, often 1/4 to 1 in, so the picture appears to sit in the optical center. It also helps with tall pieces that would otherwise look like they are sliding down.
How wide should borders be?
Wide borders look more refined than narrow ones. Common advice is at least 2 in (5 cm), and borders wider than the shortest side of a small picture can give a gallery look. For a group of frames on a wall, keep the borders consistent.
Standard sizes
| Frame | Typical mat window for |
|---|---|
| 8 × 10 in | 5 × 7 print |
| 11 × 14 in | 8 × 10 print |
| 16 × 20 in | 11 × 14 print |
| 18 × 24 in | 12 × 18 print |
Cutting tips
Mark the back of the board, cut with a sharp blade in a mat cutter at 45°, and overcut the corners by a hair so they release cleanly. Use acid-free or conservation board for anything you want to keep, and hinge the art to a backing board rather than taping it to the mat.