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. For other metric prints, the mm to inches converter gives the inch sizes, or switch the calculator to centimetres.
Photograph frame sizes and their shapes
Common photo frame sizes do not share one shape, which is why a mat so often has unequal borders or why an enlargement gets cropped. The ratio column is the long side divided by the short side. A 4 × 6 print is 1.5; an 8 × 10 is 1.25. Enlarging a 4 × 6 image to fill an 8 × 10 cuts about 17% off its long side, so check the crop before you order the print, or choose an 8 × 12 print and mat it in a larger frame.
| Ratio | Window | Mat with 2″ borders | Smallest common frame | |
|---|---|---|---|---|
| 4×6 | 1.5 | 3.5×5.5 | 7.5×9.5 | 8×10 |
| 5×7 | 1.4 | 4.5×6.5 | 8.5×10.5 | 8.5×11 |
| 8×10 | 1.25 | 7.5×9.5 | 11.5×13.5 | 12×16 |
| 8.5×11 | 1.29 | 8×10.5 | 12×14.5 | 12×16 |
| 11×14 | 1.27 | 10.5×13.5 | 14.5×17.5 | 16×20 |
| 12×16 | 1.33 | 11.5×15.5 | 15.5×19.5 | 16×20 |
| 16×20 | 1.25 | 15.5×19.5 | 19.5×23.5 | 20×24 |
| 20×30 | 1.5 | 19.5×29.5 | 23.5×33.5 | 24×36 |
The last column lists the smallest frame from the common sizes on this page that the mat fits. Where it is bigger than the mat, cut the mat to the frame opening instead and switch the calculator to frame mode; the extra space goes into the borders.
How to measure the print and the frame
- Measure the paper in several places. The Library of Congress advises measuring an object in several places across each dimension to check for variation, because hand-cut and older prints are rarely perfectly square. Use the smallest reading so the overlap holds on every side.
- Decide what should show. For overmatting, the Library of Congress says the window edge should not fall directly on the edge of the object. For a photo with a white border, measure the image area you want visible and treat the border as extra overlap.
- Measure the frame from the back. The rabbet is the recess the glass sits in. Its width and height are the frame opening this calculator asks for. Check it with a tape measure rather than relying on the size printed on the packaging.
- Leave play. NEDCC says the window mat and backing board should be the same size and fit the frame with 1/16 to 1/8″ of total play, which is why frame mode shows the opening minus 1/16″ as a cutting size.
Bottom weighting: choosing the percentage
The National Park Service describes a bottom margin about 30% wider as the more conventional layout, and the Library of Congress notes that its own storage mats are usually centred, with bottom weight added when appearance calls for it. So there is no single correct figure: 30% is the calculator's default, and you can enter any percentage. Here is an 11 × 14 print (¼″ overlap) in a 16 × 20 frame at four settings; the side borders stay at 2.75″ throughout.
| Bottom wider by | Top | Bottom | Top (nearest 1/16″) | Bottom (nearest 1/16″) |
|---|---|---|---|---|
| 0% | 3.25″ | 3.25″ | 3 1/4″ | 3 1/4″ |
| 15% | 3.023″ | 3.477″ | 3″ | 3 1/2″ |
| 30% | 2.826″ | 3.674″ | 2 13/16″ | 3 11/16″ |
| 50% | 2.6″ | 3.9″ | 2 5/8″ | 3 7/8″ |
For cutting with an inch ruler, the calculator shows the nearest 1/16″ next to every decimal inch. Rounding each border to the nearest sixteenth changes it by at most 1/32″, far less than the play the frame needs anyway. To add or subtract fractional measurements by hand, the mixed fraction calculator helps.
Outside frame size and moulding length
If you are building or ordering a frame, enter the moulding width: the distance from the rabbet edge to the outside edge of the moulding. For a mitred frame, each side's long point is the opening plus twice the moulding width, so the outside size is the opening + 2 × width, and all four sides together are the opening's perimeter + 8 × width. An 11 × 14″ opening with 1½″ moulding gives a 14 × 17″ frame and 62″ (5.17 ft) of moulding. That is the net length; buy extra for saw kerf, trimming the first cut square, and matching the grain or pattern at corners. The linear feet calculator adds up several frames.
Cutting the mat, step by step
The National Park Service's Conserve O Gram 13/1 sets out the museum method. Cut a bevelled window with a commercial or hand-held mat cutter guided by a steel straight edge, on a flat surface covered with cardboard, and change blades as soon as they begin to dull; the goal is a clean cut with no overcutting at the corners. Cut the back board to the same outside dimensions as the window mat, then hinge the two together along the top or left edge (conventionally the longest edge) with linen or polyester fibre tape, which goes on the boards and never on the object. Practise on scrap board first: a window cut too large cannot be fixed, while one cut slightly small can be recut.
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.
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 size mat for a 5×7 photo in an 8×10 frame?
Cut the mat to the 8 × 10 frame opening and the window to 4.5 × 6.5″ (a ¼″ overlap). That leaves 1.75″ at the sides and 1.75″ top and bottom, so the borders are even all round.
What size mat for a 4×6 photo?
The window is 3.5 × 5.5″ with a ¼″ overlap. In a 5 × 7 frame the borders are only 0.75″ wide, under the 1″ handling margin the Library of Congress suggests, so an 8 × 10 frame (2.25″ sides, 2.25″ top and bottom) usually looks better.
How do I measure a frame for a mat?
Take the frame apart and measure the opening at the back, where the glass sits (the rabbet), not the outside of the moulding or the visible front opening. Measure in two or three places across each dimension, as the Library of Congress advises for artwork, and use the smallest reading.
How much moulding do I need to build a frame?
For a mitred frame, add eight times the moulding width to the perimeter of the opening. An 11 × 14″ opening with 1½″ moulding needs 2 × (11 + 14) + 8 × 1.5 = 62″, or 5.17 feet, before any allowance for saw cuts and mistakes.
What size frame do I need for an 8×10 with a mat?
With a ¼″ overlap and 2″ borders the mat is 11.5 × 13.5″, which is not a stock size. The smallest common frame that holds it is 12 × 16″; an 11 × 14 frame gives slightly narrower borders of 1.75″ and 2.25″.
Can I use centimetres for the mat calculator?
Yes. Switch Units to centimetres and every input and result uses cm. A ¼″ overlap is 0.635 cm, so about 0.6 cm (6 mm) per side is the metric equivalent.
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.