Picture mat size and border planner
Cut the opening 7.50 in by 9.50 in in a mat 13.50 in by 15.50 in, with 3.00 in at the sides.
Result
Opening width
in
Opening height
in
Mat width
in
Mat height
in
| Artwork width | 8.00 in |
|---|---|
| Artwork height | 10.00 in |
| Opening width (artwork less the overlaps) | 7.50 in |
| Opening height (artwork less the overlaps) | 9.50 in |
| Border at each side | 3.00 in |
| Border at the top | 3.00 in |
| Border at the bottom | 3.00 in |
| Mat width (opening plus the two side borders) | 13.50 in |
| Mat height (opening plus top and bottom) | 15.50 in |
How it works
A mat covers a thin edge of the artwork so it cannot slip, so the opening is the artwork minus the overlap on each side. The border is the visible mat from the opening to the mat's edge. The mat is the opening plus a border on each side. Framers often make the bottom border a little wider than the others, a weighted bottom, because an equal border looks as if it sags. You can start from the border you want, or from a ready-made mat to see the opening and the borders it leaves. Ready-made mats come in sizes such as 8x10 and 11x14 (8 × 10 and 11 × 14 inches): enter your artwork and the mat to find the mat opening size for an 8x10 photo in an 11x14 frame.
Formula
opening_w = art_w - 2 x overlap and opening_h = art_h - 2 x overlap. Mode border: the border is measured from the opening to the edge of the mat. side = top = border and bottom = border + bottom_extra; mat_outer_w = opening_w + 2 x side; mat_outer_h = opening_h + top + bottom. Mode mat (a ready-made mat of mat_w x mat_h, turned to the artwork's orientation): side = (mat_w - opening_w) / 2; top = (mat_h - opening_h - bottom_extra) / 2; bottom = top + bottom_extra. third_rule = min(art_w, art_h) / 3; border_share = side / min(art_w, art_h). w_border_thin appears when border_share < 1/3, w_border_wide when border_share > 1/2 (with a 1e-9 tolerance).
Worked example
The tool opens with this example; the result above is computed from it.
| Input | Value |
|---|---|
| You know | The border I want |
| Artwork width | 8 in |
| Artwork height | 10 in |
| Overlap on each side | 0.25 in |
| Border width | 3 in |
| Extra at the bottom | 0 in |
| Opening width | 7.50 in |
| Opening height | 9.50 in |
| Mat width | 13.50 in |
| Mat height | 15.50 in |
Common cases
| Case | Starting values | Opening width | Opening height | Mat width | Mat height |
|---|---|---|---|---|---|
| Eight by ten print in an eleven by fourteen mat | Artwork width 8 in; Artwork height 10 in; Overlap on each side 0.25 in; Mat width 11 in; Mat height 14 in; Extra at the bottom 0 in | 7.50 in | 9.50 in | 11.00 in | 14.00 in |
| Photo with a weighted bottom | Artwork width 8 in; Artwork height 10 in; Overlap on each side 0.25 in; Border width 3 in; Extra at the bottom 0.5 in | 7.50 in | 9.50 in | 13.50 in | 16.00 in |
| Small print with wide borders | Artwork width 4 in; Artwork height 6 in; Overlap on each side 0.25 in; Border width 2 in; Extra at the bottom 0 in | 3.50 in | 5.50 in | 7.50 in | 9.50 in |
| Large print with a proportionate border | Artwork width 30 in; Artwork height 40 in; Overlap on each side 0.25 in; Border width 6 in; Extra at the bottom 0.5 in | 29.50 in | 39.50 in | 41.50 in | 52.00 in |
Questions and answers
What mat opening do I need for an 8 × 10 photo in an 11 × 14 mat?
With an overlap of 0.25 in the opening is 7.50 in by 9.50 in. In a 11 in by 14 in mat that leaves 1.75 in at the sides and 2.25 in at the top and bottom.
How much wider should the bottom of a mat be?
Framers add a little: here the sides and top are 3 in and the bottom is 3.50 in, which is 0.5 in more. A mat for this 8 in by 10 in picture is 13.50 in by 16.00 in.
Assumptions and limits
- The overlap is subtracted from the artwork on every side, so the opening is smaller than the artwork in both directions.
- The weighted bottom is added to the bottom border only; the top and sides stay equal.
- The one-third rule and its limits are one framer's guidance, not a standard; contemporary framing often prefers equal borders.
- The mat size is also the size of the glass and backing it sits on, if you buy a frame to take it.
The result is an estimate for planning, not professional advice.
Sources
- Custom Picture Frames, border width proportions: one-third rule, half for small art, weighted bottoms of half to one inchretrieved 2026-10-13
- Profile Products mat border calculator: the mat overlaps the artwork by 0.5 cm on each sideretrieved 2026-10-13
Last updated:
Method in depth: Mat borders, overlap and a weighted bottom
Cite this page: Frame Kit, “Picture mat size and border planner”, https://frame-kit.pages.dev/framing-and-hanging/picture-mat-size-and-border-planner/ (updated 13 October 2026).
Found a mistake? Email handycalc.sites+framekit@gmail.com.