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How to choose a picture mat and border
A mat is a thin paper-based board with a bevelled window that keeps the artwork off the glass; Wikipedia says that contact can cause condensation, mould and mildew, and the Canadian Conservation Institute recommends four-ply archival mat board, acid-free or buffered. Borders are usually equal on all sides, or bottom-weighted with a larger bottom margin. For equal borders subtract the opening from the outer size and halve it.
Diagram: mat anatomy. Schematic, not to scale.
Border arithmetic for common sizes
Margin each side = (outer width minus opening width) divided by two; likewise for height. Inches first, centimetres second. This is arithmetic on example sizes, not a rule for your picture.
| Opening (in) | Outer mat size (in) | Side margin (in) | Top and bottom margin (in) | Side / top-bottom (cm) |
|---|---|---|---|---|
| 5 x 7 | 8 x 10 | 1.5 | 1.5 | 3.8 / 3.8 |
| 8 x 10 | 11 x 14 | 1.5 | 2 | 3.8 / 5.1 |
| 8 x 12 | 12 x 16 | 2 | 2 | 5.1 / 5.1 |
| 12 x 18 | 16 x 24 | 2 | 3 | 5.1 / 7.6 |
| 16 x 20 | 20 x 24 | 2 | 2 | 5.1 / 5.1 |
| 11 x 14 | 16 x 20 | 2.5 | 3 | 6.4 / 7.6 |
Choosing
| Choice | What to know | Source |
|---|---|---|
| Equal margins | The most common layout | WMAT |
| Bottom-weighted | Larger bottom than top and sides; counters the tendency to see an image's centre as higher than its physical centre | WMAT |
| Material | Acid-free or conservation board; older wood-pulp mats can cause mat burn, brown marks that spread | WMAT, CCIP |
| Attachment | Japanese paper hinges, not glue or tape on the artwork | CCIP |
See mat board for the grades from museum board down to acid-free lined board. This page is general framing guidance; check what is in the product you order.
Sources
- Wikipedia: Mat (picture framing) (fetched 2026-11-12)
- Canadian Conservation Institute (Government of Canada): basic care, works of art on paper (fetched 2026-11-12)
Figures from these pages are paraphrased and re-checked on 2026-11-12. Where two sources disagree, both figures are shown. Hardware and wall products vary, so follow the pack instructions.
Related pages
- Mat board
- Photo frame sizes in inches and cm
- Hanging and display guides
- Care guide: keep your piece looking good
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