Standard picture frame sizes, mats and metric

The size on the label, 11x14 for example, is the to-fit size: the glass, backing board and artwork the frame takes. The moulding adds to that on every side, so the frame on the wall is larger. A lip on the inside edge of the moulding, called the rabbet, holds the glass in and covers about 1/4 inch of each edge, which means an 18x24 frame shows 17.5 by 23.5 inches of whatever is inside it.

Outside size equals the frame size, plus twice the moulding width, minus twice the lip. An 11x14 frame with 1-inch moulding and a 1/4-inch lip measures 12.5 by 15.5 inches outside. A mat goes the other way: its window is cut smaller than the print so the print cannot fall through, and pre-cut mats sold for an 8x10 commonly have a 7.5 by 9.5 inch window.

With a mat, the frame size is the mat window plus the border on both sides. That is why the common pairings jump a size or two: a 5x7 print goes in an 8x10 frame, an 8x10 in an 11x14, an 11x14 in a 16x20. The pairing table on this page shows the border each one leaves once the window overlaps the print by 1/4 inch: from a narrow 0.75 inch for a 4x6 in a 5x7 frame to more than 5 inches for an 8x10 in a 16x20.

Borders come out unequal whenever the print and frame have different shapes. An 8x10 in an 11x14 frame gets 1.75 inches at the sides and 2.25 at the top and bottom. A 24x36 poster in a 30x40 frame goes the other way, with 3.25 inches at the sides and 2.25 above and below. Some framers cut the bottom border a little wider on purpose, a practice called bottom weighting. The calculator shows the borders any frame will leave before you buy it.

A print keeps the whole photo only when the two share a shape. Full-frame and APS-C camera sensors are 3:2, the 36 by 24 millimetre shape of 35 mm film, so a camera photo fits 4x6, 12x18, 16x24, 20x30 and 24x36 exactly. Phone cameras usually default to 4:3, which matches 6x8, 9x12, 12x16, 18x24 and 30x40.

Everything else crops. A 3:2 photo filled onto 8x10 paper is first scaled to 8 by 12 inches, then 2 inches come off its length, 16.7 percent of the picture. A 4:3 phone photo loses 6.3 percent on the same paper. If the crop would take something that matters, print the whole image smaller on the paper and let the mat hide the white margin, or pick a size from the photo's own ratio family.

The A sizes are defined by ISO 216: A0 has an area of one square metre, and each size is the previous one halved across its long side, so every A sheet has the same 1 to √2 shape. A4 is 210 by 297 millimetres and A3 is 297 by 420. Because the shape never changes, any A-size print scales to any other A size without cropping, which no pair of common inch sizes can claim.

The inch and metric systems almost meet in several places, and almost is the problem. US Letter is 5.9 millimetres wider and 17.6 millimetres shorter than A4, so neither fits the other's frame. A 12x16 inch print is 4.8 by 6.4 millimetres too big for a 30 x 40 cm frame, and a 16x20 is 6.4 by 8 millimetres too big for a 40 x 50. A 24x36 poster does go in a 61 x 91.5 cm maxi frame.

Turn the frame over and measure the glass or the backing board, not the moulding and not the opening on the front. That measurement is the frame size. The front opening is smaller by the lip on each side, and measuring it is the usual reason a print bought to fit turns out half an inch too small all round.

For the outside size, measure edge to edge across the back of the moulding. Note the moulding width as well, the face from the inner edge to the outer edge, because it decides how much wall each frame takes. When the plan is a gallery wall, the outside size is the one that matters: gaps are measured between outside edges, and two frames with the same to-fit size can differ by inches on the wall.

Mixed walls hold together best when they use two or three sizes rather than one of everything. A flexible starting set is two 11x14, two 8x10 and one or two 5x7 frames, with a 16x20 or 20x24 added as an anchor on a sofa-width wall. Matting different prints into the same frame size is the other route to calm: the outlines repeat while the pictures change.

Before buying, enter each frame's outside size in the planner rather than its label size, because the moulding is what touches the gap. Then check the arrangement width against the wall. The size guide covers how big the whole arrangement should be for a given wall or sofa, and the frame set guide covers mixing finishes and profiles.