A mat window should be slightly smaller than your print: print size minus twice the overlap, usually ¼ inch per side. An 8 × 10 print gets a 7½ × 9½ inch window. In an 11 × 14 frame that leaves 1.75″ borders at the sides and 2.25″ top and bottom. This calculator works out the window, the mat size or the borders for any print and frame.
How to use the mat calculator
- Choose what you know: the border width you want, or the frame size you already have.
- Measure the print itself, not the image area, and enter width and height in inches.
- Set the overlap. ¼″ is the common default; enter 0 if the mat should just meet the print edge.
- Enter the border width, or the frame's opening size (the rabbet), not its outside size.
- For a frame, choose even or bottom-weighted borders.
- Cut the window to the result, and the outer mat to the frame opening or the mat size shown.
How the mat window and borders are calculated
- Window = print width − 2 × overlap, by print height − 2 × overlap.
- Mat size (border mode) = window + 2 × border in each direction.
- Side borders (frame mode) = (frame width − window width) ÷ 2.
- Top and bottom, even = (frame height − window height) ÷ 2.
- Top and bottom, weighted: the vertical space is split so the bottom is 1.3 × the top, following the National Park Service's "about 30% wider" guidance. Top = vertical space ÷ 2.3.
Worked examples
- Border you choose: an 8 × 10 print, ¼″ overlap and 2″ borders gives a 7.5 × 9.5″ window and a 11.5 × 13.5″ mat. That is not a stock frame size, so either order a custom frame or switch to frame mode.
- Frame you have: the same print in an 11 × 14 frame gives 1.75″ side borders and 2.25″ top and bottom.
- Bottom-weighted: an 11 × 14 print in a 16 × 20 frame has a 10.5 × 13.5″ window, 2.75″ sides, a 2.83″ top and a 3.67″ bottom.
- Floating: a 5 × 7 print with a ¼″ reveal needs a 5.5 × 7.5″ window. Enter the overlap as 0 in the calculator and add the gap yourself: window = print + 2 × gap.
Common print and frame pairings (¼″ overlap)
| Frame | Window | Sides | Top / bottom | |
|---|---|---|---|---|
| 4×6 | 8×10 | 3.5×5.5 | 2.25″ | 2.25″ |
| 5×7 | 8×10 | 4.5×6.5 | 1.75″ | 1.75″ |
| 5×7 | 11×14 | 4.5×6.5 | 3.25″ | 3.75″ |
| 8×10 | 11×14 | 7.5×9.5 | 1.75″ | 2.25″ |
| 8×10 | 16×20 | 7.5×9.5 | 4.25″ | 5.25″ |
| 11×14 | 16×20 | 10.5×13.5 | 2.75″ | 3.25″ |
| 16×20 | 20×24 | 15.5×19.5 | 2.25″ | 2.25″ |
Common print sizes: 4×6, 5×7, 8×10, 8.5×11, 11×14, 12×16, 16×20, 18×24, 20×30, 24×36 inches. These are trade sizes, not a formal standard. Notice that in several pairings the side and top borders differ, because print and frame proportions rarely match exactly.
A4, A3 and A5 prints in inch frames
ISO 216 defines A-series paper in millimetres. Converted at 25.4 mm per inch with a ¼″ overlap:
| Paper | Size (in) | Window | In frame | Sides | Top / bottom |
|---|---|---|---|---|---|
| A5 (148×210 mm) | 5.83×8.27 | 5.33×7.77 | 8×10 | 1.34″ | 1.12″ |
| A4 (210×297 mm) | 8.27×11.69 | 7.77×11.19 | 11×14 | 1.62″ | 1.4″ |
| A3 (297×420 mm) | 11.69×16.54 | 11.19×16.04 | 16×20 | 2.4″ | 1.98″ |
US Letter paper (8.5 × 11 in) is in the print list above; enter it in the calculator with your frame size.
How wide should mat borders be?
The National Park Service's Conserve O Gram 13/1 gives 2 to 7 inches as an adequate margin, scaled to the object: about 2–3 inches for a 5 × 7 and 4–5 inches for a 24 × 30. When a frame or storage box fixes the outside size, it suggests placing the object for a balanced appearance within the space available. The Library of Congress adds that the outer mat should leave at least 1 inch per side for handling.
The NPS describes two ways to proportion a mat: all four margins equal, or the more conventional bottom-weighted layout with the bottom about 30% wider. Pick the frame-mode option that matches the look you want.
Choosing mat board and glazing
- Board: NEDCC recommends 100% cotton rag for window mats and backing. The NPS rates cotton board buffered with calcium carbonate as best, and lignin-free buffered woodpulp board as acceptable.
- Avoid cheap cores: the NPS warns that boards with a groundwood core release acids along the bevel, causing the yellow line known as acid mat burn.
- Thickness: NEDCC says four-ply board may suffice, but thicker or laminated mats are needed for large sheets, cockled paper or raised media, so the glazing never touches the object.
- Glazing: NEDCC advises UV-filtering glazing that blocks at least 99% of UV, and never conventional acrylic over pastel, charcoal or other friable media, because of its static charge.
- Hinges: NEDCC and the NPS both describe Japanese paper hinges with wheat starch paste. The NPS suggests hinges 1–2″ wide, attached to the back of the top edge, with three to five small hinges rather than two wide ones on larger pieces.
Common mistakes
- Measuring the frame's outside edge. Use the opening at the back (the rabbet), then allow NEDCC's 1/16–1/8″ of total play.
- Cutting the window to the print size. With no overlap the print can fall through; subtract the overlap first.
- Assuming the image fills the print. Many photo prints have a white border. Measure what you want to show and set the overlap accordingly.
- Forgetting proportions. An 8 × 10 print in a 16 × 20 frame gives unequal side and top borders; the table above shows this.
Buying moulding or trim by length for a custom frame? The linear feet calculator adds up the pieces.
Frequently asked questions
How big should a mat opening be?
Slightly smaller than the print so the mat overlaps its edges. The Library of Congress notes that ¼ inch per side is often considered standard, so an 8 × 10 print gets a 7½ × 9½ inch window.
What size mat for an 8×10 in an 11×14 frame?
With a ¼″ overlap the window is 7.5 × 9.5″, leaving 1.75″ borders at the sides and 2.25″ at the top and bottom. The mat itself is cut to the full 11 × 14″ frame opening.
What size mat for an 11×14 print in a 16×20 frame?
The window is 10.5 × 13.5″. Even borders are 2.75″ at the sides and 3.25″ top and bottom; bottom-weighted, the top is 2.83″ and the bottom 3.67″.
What is a bottom-weighted mat?
A mat with a wider bottom border, which makes the picture look centred when it hangs. The National Park Service describes the conventional practice as equal top and side margins with a bottom margin about 30% wider; this calculator uses bottom = 1.3 × top.
How wide should mat borders be?
The National Park Service gives 2 to 7 inches as an adequate margin, with larger objects getting larger margins: about 2–3 inches for a 5 × 7 and 4–5 inches for a 24 × 30. The Library of Congress suggests at least 1 inch on each side for handling.
How do I float a print instead of overlapping it?
Make the window bigger than the print so its edges show. The Library of Congress says the gap is often ⅛ to ¼ inch for smaller pieces. For a 5 × 7 print with a ¼″ gap all round, the window is 5.5 × 7.5″.
How much smaller than the frame should the mat be?
NEDCC recommends that the window mat and backing board fit the frame with 1/16 to 1/8 inch of total play, so they are not forced into the rabbet.
What mat board should I use for a photo or artwork?
NEDCC recommends 100% cotton rag board. Most boards are buffered with calcium carbonate, but NEDCC notes unbuffered boards exist for sensitive photographs such as cyanotypes, some albumen prints and many color prints.
Are photo print and frame sizes standardized?
Not by any public standard we could find. Sizes such as 4×6, 5×7, 8×10 and 11×14 inches are common trade sizes, so measure your print and frame opening. A-series paper (A4 = 210 × 297 mm) is defined by ISO 216.
What does the mat overlap mean?
It is how far the mat covers the print on each side. The window equals the print size minus twice the overlap, so a ¼″ overlap makes the window ½″ smaller than the print in each direction.
Sources & method
- Library of Congress — Matting and hinging works of art on paper: planning (window overlap, float gap, outer margin)
- National Park Service — Conserve O Gram 13/1, window mats for paper objects (margins, bottom weighting, hinges)
- NEDCC — Preservation leaflet 4.10, matting and framing for works on paper and photographs
- ISO 216:2007 — A-series paper sizes, Table 1 (official preview PDF)
Results are estimates for general information. Found an error? It helps everyone — see our methodology.