Mat width and the frame size it produces

Updated August 17, 2026

A mat is not decoration. It is the layer that absorbs the difference between the shape of a print and the shape of a frame, and its width is arithmetic before it is taste: pick the border, and the frame size is already decided.

The short answer

Frame size equals artwork size plus twice the mat border, in each direction. A 20 × 25 cm print with a 5 cm mat needs a 30 × 35 cm frame; an 8 × 10 inch print with a 2 inch mat needs 12 × 14.

In practice nobody starts from a blank frame size, because frames are sold in standard sizes. So the sum runs backwards: the frame is fixed, and what the arithmetic gives you is the border — which is usually not the same on all four sides.

Three things are called “frame size”, and they are different numbers

Most of the confusion in this topic is vocabulary rather than measurement.

• The frame size, which is the size of the glass and of the mat behind it. A 30 × 40 cm frame takes a 30 × 40 cm sheet with no mat at all.
• The window opening, cut 3–6 mm (an eighth to a quarter inch) smaller than the artwork on each side so the print cannot fall through. A ready-made mat for an 8 × 10 photograph has an opening near 7.5 × 9.5 inches for exactly this reason.
• The outer dimension, which adds the moulding — 2 cm of wood on each side turns a 30 × 40 frame into 34 × 44 on the wall.

Buy by the first number, cut by the second, hang by the third. A listing that says “30 × 40” without saying which of the three it means is the sentence that produces “it did not fit” messages.

How wide the mat should be

Framers work in a range rather than a formula, roughly 4 to 10 cm (1.5 to 4 inches), widening with the artwork. Taking the lower end of that range for each size gives:

The two halves of each cell are parallel tracks, not conversions of each other: A3 is not 12 × 16 inches, and a frame that fits one will not fit the other. It is also why only two entries in the right-hand column — 8 × 10 and 24 × 30 inches — are frames anyone actually stocks. Computing forwards from a mat width lands on custom framing almost every time, which is the practical argument for running the sum backwards from a frame you can buy.

Two limits bracket the range. Below about 2.5 cm the mat stops reading as a deliberate border and starts reading as a printing error, and it holds the sheet less securely. Above roughly a third of the artwork's short side the piece begins to look lost in its own frame — with an exception for small works hung deliberately in large mounts, which is a style rather than an accident.

The one rule worth treating as fixed: the mat should be wider than the moulding. A 3 cm frame around a 3 cm mat gives two competing borders of equal weight, and the eye reads the frame as the subject.

Equal borders are the exception, not the target

A mat has borders of equal width only when the print and the frame share a ratio. They usually do not, and that is what the mat is for — it absorbs the difference between ratio families instead of cropping the image.

The last row is the test worth memorising: side borders must never come out wider than the top and bottom. Mats are wider vertically or equal, never horizontally — when the arithmetic says otherwise, the print is in a frame from the wrong ratio family and no mat width will rescue it. A 12 × 18 print needs 16 × 24 or 18 × 24, not 16 × 20.

Weight the bottom

Traditional framing adds 10–20 % to the bottom border, because the optical centre of a hanging object sits slightly above its geometric centre and an evenly matted piece looks like it has slipped downwards in the frame.

On a 7 cm mat that means sides and top at 7 cm and the bottom at 8 — a difference nobody consciously notices and everybody sees. It matters most on large work at eye level, and can be skipped on small pieces where the whole border is under 4 cm.

A second mat, and what it is for

A double mat shows a thin strip of a second colour along the window edge, usually 3–6 mm of it. The strip does one job: it separates the artwork's own colours from the mat's, which is worth doing when both are pale and the print's edge would otherwise dissolve into the board.

The inner colour is picked from inside the artwork rather than from the room — a muted version of a colour already present. Anything brighter than the print competes with it, and a bright strip around a quiet piece is the most common way a double mat goes wrong.

If you sell the print, the mat is a description problem

Buyers who intend to frame need three facts, and a listing that omits them generates messages instead of orders:

1. Paper size and image size, separately, if there is a white border printed around the artwork. A 30 × 40 sheet with a 3 cm printed margin is a 24 × 34 image, and it will be matted as one.
2. Whether the stated size includes a mat. It normally does not, and saying so costs one line.
3. That the frame is not included, if it is not — obvious to the seller, not always to the buyer looking at a framed photograph.

The listing photo is where this gets settled, because the buyer reads the picture before the text. What each image in the set has to prove is covered in what belongs in a listing's photos.

Seeing the border before cutting the board

Mat width is the kind of decision that looks obvious in numbers and different on a wall, and mat board is bought by the sheet. Placing the artwork in a photographed room with the mat and frame applied shows the proportion at a glance — the mockup generator does it in the browser, and how the compositing works covers the mat, frame and shadow controls in order.

The numbers here are framing conventions, not standards: shops differ, and conservation framing for original work has its own requirements around materials and mounting that no mat width table covers.

Questions about this