Picture Frame Mat Calculator
Mat window size and border widths for your artwork, including the weighted bottom margin that stops a framed picture looking like it is sliding downwards.
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Results
Front view, drawn to scale. The bottom border is deliberately deeper than the top.
Working
- window = artwork 8 x 10 less 0.5 overlap = 7.5 x 9.5 in
- board = frame opening = 16 x 20 in
- side border = (16 - 7.5) / 2 = 4.25 in
- vertical margin = 20 - 9.5 = 10.5 in
- top = 10.5 / (2 + 0.2) = 4.773 in
- bottom = 4.773 x 1.2 = 5.727 in
Quick answer
Cut the window an eighth to a quarter inch smaller than the artwork on each edge so the art has something to sit behind. Subtract the window from the frame opening to get the total margin, split the sides equally, and give the bottom about twenty per cent more than the top so the picture looks centred rather than sinking.
window = art - 2 x overlap; top = (board h - window h) / (2 + weight); bottom = top x (1 + weight)
Worked example
An 8 by 10 in print in a 16 by 20 in frame, quarter inch overlap, bottom weighted twenty per cent.
Working
- window = artwork 8 x 10 less 0.5 overlap = 7.5 x 9.5 in
- board = frame opening = 16 x 20 in
- side border = (16 - 7.5) / 2 = 4.25 in
- vertical margin = 20 - 9.5 = 10.5 in
- top = 10.5 / (2 + 0.2) = 4.773 in
- bottom = 4.773 x 1.2 = 5.727 in
The side borders come out at 4-1/4 in, the top at 4-3/4 and the bottom at 5-3/4. Measured with a rule that looks wrong. On the wall it is the only version that looks right, because the eye reads the optical centre as slightly above the true one.
Standard mat sizes for common frames
Quarter inch overlap on each edge and a twenty per cent weighted bottom, which is the standard framing convention.
| Artwork | Frame | Window | Sides | Top | Bottom |
|---|---|---|---|---|---|
| 5 x 7 in | 8 x 10 in | 4-1/2 in x 6-1/2 in | 1.8 in | 1.6 in | 1.9 in |
| 8 x 10 in | 11 x 14 in | 7-1/2 in x 9-1/2 in | 1.8 in | 2 in | 2.5 in |
| 8 x 10 in | 16 x 20 in | 7-1/2 in x 9-1/2 in | 4.3 in | 4.8 in | 5.7 in |
| 11 x 14 in | 16 x 20 in | 10-1/2 in x 13-1/2 in | 2.8 in | 3 in | 3.5 in |
| 12 x 16 in | 18 x 24 in | 11-1/2 in x 15-1/2 in | 3.3 in | 3.9 in | 4.6 in |
| 16 x 20 in | 24 x 30 in | 15-1/2 in x 19-1/2 in | 4.3 in | 4.8 in | 5.7 in |
Frame sizes are the opening, which is what the mat board is cut to. A frame sold as 16 by 20 in usually has an opening about an eighth under that, so measure the rabbet.
At a glance
| What it does | Window size, window position and the three border widths for a mat |
|---|---|
| Inputs needed | Artwork size, overlap, frame opening or desired border, bottom weighting |
| Main output | Window dimensions and border widths |
| Formula | window = art - 2 x overlap; bottom border = top x (1 + weighting) |
| Category | Crafts & Hobbies |
| Cost | Free, no sign-up, no download |
How to use the Picture Frame Mat Calculator
- Measure the artwork itself, including any part you intend the mat to cover.
- Set the overlap. A quarter inch each edge is standard; go to an eighth only if the image runs close to the sheet edge.
- Measure the frame's rabbet opening rather than trusting the size printed on the box.
- Leave the bottom weighting at twenty per cent unless the frame is very small.
- Cut the window from the back of the board with a bevel cutter, working to the window position given from the top left corner.
Frequently asked questions
How much smaller should a mat window be than the artwork?
An eighth to a quarter inch smaller on each edge. That overlap is structural, not decorative: without it the art has nothing to rest against and drops through the window as soon as the frame is tilted.
Why is the bottom mat border wider?
Because the eye reads the optical centre as slightly above the geometric one. With equal borders a framed picture looks as though it has slipped downwards. Adding fifteen to twenty per cent to the bottom corrects it, and the effect is stronger the larger the frame.
How wide should a mat border be?
At least an inch and a half, or it stops reading as a deliberate mat. Two to three inches suits most domestic work, and wider still suits small pieces where the mat is doing the work of drawing the eye in.
What size mat do I need for an 8 by 10 print?
In a 16 by 20 in frame with a quarter inch overlap, the window is 7-1/2 by 9-1/2 in, leaving 4-1/8 in side borders, 4-7/16 in at the top and 5-5/16 in at the bottom with a twenty per cent weighting.
What is a double mat reveal?
The strip of the lower mat showing around the window, usually a quarter inch. The lower mat's window is cut smaller than the top one's by twice the reveal, so it peeks out evenly all the way round.
Does mat board quality matter?
For anything you want to keep, yes. Ordinary wood pulp board turns acidic and burns a brown line into the art along the bevel line within a few years. Cotton rag or alpha cellulose board is inert and does not.
Window and border figures are exact arithmetic. Frames vary: an opening sold as a nominal size is often an eighth of an inch under, so measure the rabbet and cut a test piece from scrap before committing good board.