How to use the Frame & Mat Size Calculator
- Pick what you already know. Choose Set the border if you want a particular mat width and need to know which frame to buy. Choose Fit a frame if you already own the frame and need the mat to fill it.
- Enter the artwork size and the overlap. Measure the paper or printed image, not the frame. Type fractions the way a tape measure reads (
10 3/4,3/8) or switch to cm. - Set the border or the frame. In border mode, type one width for the top and sides. In frame mode, pick a ready-made size or choose Custom size…. Tick Bottom-weighted to add extra width under the art.
- Read the result and the preview. The big number is the frame size — also the size of the mat board, glass and backing. The to-scale drawing shows every border and the window. If a ready-made frame is close, the note underneath lists the borders you’d get in it; press Use to switch to that frame.
- Take the numbers to the cutter. Copy cut list gives the window, borders, window position and frame size as text. Download SVG saves the diagram.
How mat and frame sizes fit together
Everything in a frame stacks inside the rabbet, the recessed step on the back of the moulding. Glass, mat, art and backing are cut to the same outer size, and that is the size the frame is sold as: a “16 × 20 frame” takes a 16 × 20 inch sheet of glass.
The mat is a board with a bevel-cut hole, the window. The window is slightly smaller than the artwork so the board overlaps the edges and holds the paper flat. An overlap of 1/8 to 1/4 inch per side is standard. The smaller figure shows more of the image; the larger one forgives paper that isn’t perfectly square. To show the deckled edge of a watercolor sheet, use a negative overlap: the window becomes larger than the art and leaves a gap, called a float or reveal.
The arithmetic is short:
- Window = artwork − 2 × overlap, in each direction.
- Frame size = window + both side borders across, and window + top + bottom border down.
- Visible border = border − the part hidden by the rabbet lip, usually about 1/4 inch (6 mm).
Take an 11 × 14 inch print with a 1/4 inch overlap. The window is 10 1/2 × 13 1/2 inches. With 2 1/2 inch borders and an extra 1/2 inch at the bottom, the mat comes out at 15 1/2 × 19 inches. That isn’t a stock size, so you’d order a custom frame or move up to a 16 × 20, where the same window leaves 2 3/4 inch sides, a 3 inch top and a 3 1/2 inch bottom. A 14 × 18 works for a tighter look: 1 3/4 inch sides, 2 inch top, 2 1/2 inch bottom.
Ready-made frame pairings, worked out
Common print sizes in common frames, with a 1/4 inch overlap and centered top and bottom borders. For a 1/2 inch bottom weight, take 1/4 inch off the top figure and add it to the bottom.
| Artwork | Frame | Window | Sides | Top & bottom |
|---|---|---|---|---|
| 5 × 7 in | 8 × 10 in | 4 1/2 × 6 1/2 in | 1 3/4 in | 1 3/4 in |
| 8 × 10 in | 11 × 14 in | 7 1/2 × 9 1/2 in | 1 3/4 in | 2 1/4 in |
| 8 × 10 in | 12 × 16 in | 7 1/2 × 9 1/2 in | 2 1/4 in | 3 1/4 in |
| 8.5 × 11 in | 11 × 14 in | 8 × 10 1/2 in | 1 1/2 in | 1 3/4 in |
| 11 × 14 in | 16 × 20 in | 10 1/2 × 13 1/2 in | 2 3/4 in | 3 1/4 in |
| 18 × 24 in | 24 × 30 in | 17 1/2 × 23 1/2 in | 3 1/4 in | 3 1/4 in |
| A4 (21 × 29.7 cm) | 30 × 40 cm | 19.8 × 28.5 cm | 5.1 cm | 5.75 cm |
Ready-made frames rarely give perfectly even borders, because frame proportions and print proportions don’t match. An 8 × 10 in an 11 × 14 has 1/2 inch more mat at top and bottom than at the sides, and most viewers read that as intentional. The “next size up” habit also runs thin fast: letter paper in an 11 × 14 leaves 1 1/2 inch sides, only 1 1/4 inch once the lip takes its share.
Mistakes that waste a sheet of mat board
Measuring the frame from the front. You need the rabbet size — measure across the back, inside the recess. The front opening is smaller by the lip on each side; the outside is larger by the moulding.
Centering the window vertically. A perfectly centered window looks as if it has slipped down, so framers make the bottom border slightly wider. A 1/4 to 1/2 inch difference is typical on small and medium pieces.
Cutting a custom frame to the exact mat size. Add about 1/8 inch (3 mm) to each rabbet dimension. Boards and glass are never perfectly square, and a frame with zero slack bows or cracks the glass in humid weather.
Tips from practice
Make the border noticeably wider than the moulding. A 2 inch mat inside a 2 inch moulding reads as two competing stripes. When the mat clearly wins, the eye moves past the frame to the art.
Plan cuts on full sheets. Mat board commonly comes in 32 × 40 inch sheets. Six 11 × 14 mats fit on one, but only four 16 × 20s — useful to know when you’re matting a whole series for a show.
Check the stack depth before buying a slim frame. Picture glass is about 2 mm, a 4-ply mat about 1.5 mm and 3/16 inch foam board close to 5 mm. Add a second mat and many shallow frames run out of room.
Keep a series to two or three frame sizes. Hanging is easier and cheaper. Use the Aspect Ratio Calculator to plan prints that drop into the same frames, and the DPI & Print Size Calculator to check your file before ordering.