Mat Boards in Photo Frames: What They Are and How to Size the Print

Created by: PrintForFun. Reviewed and updated by the KT Colour team, October 2026.

The short answer: a mat board is the card border between the frame and the photo. It leaves a window smaller than the frame, so the print you order must match the window plus an overlap, not the frame size. Measure the window, add about 5 mm each side for the overlap, and order that print size; or send us the frame and we cut a mat to fit any print. Use a mat for anything on a wall, skip it for small desk frames.

What a mat does

A mat, also called a mount in British English, is a sheet of thick card with a window cut in it. It sits between the glass and the print. It does three things: it gives the photo breathing room so the eye settles on it rather than the frame; it keeps the print from touching the glass, which matters in our humidity because photographic paper can stick to glass; and it hides the edge of the print, so slight trim differences never show.

Mat sizes are not standard

Frames are sold by the size of print they take without a mat. The mat window is whatever the maker decided, and it varies by brand and range. An IKEA RIBBA 30 x 40 cm frame, for example, comes with a mat for a 21 x 30 cm print, while another brand's 30 x 40 might have a window for 20 x 25 cm. There is no way around measuring.

How to measure for a print

1. Measure the window of the mat, width and height, in millimetres. This is the area you will see.
2. Add the overlap. The print must extend behind the mat so it does not fall through or show a gap. Add 5 mm to each side, so 10 mm to each dimension. A 200 x 300 mm window takes a 210 x 310 mm print.
3. Choose the nearest standard size, or a custom size. If the result is close to a standard size (that example is near A4 at 210 x 297 mm), order that and trim it to fit. If not, ask your print lab for a custom size and give the exact measurement.
4. Crop for the window, not the print. Remember that only the window will show. Faces near the edge of the photo will be behind the mat. Our Photo Crop Calculator previews the crop for any size.

Printing a mat into the print

There is a cheaper alternative for frames that came without a mat: print the photo smaller than the paper, with an even white border around it. The border does the job of a mat visually, with no card to cut. Add the border yourself in any photo editor before ordering, or ask the lab to set the paper size and border width for you.

When to use a mat

• Use one for wall frames, anything A4 and above, photographs under glass, and anything you want to look finished rather than snapshot-like. A mat 5 to 8 cm wide suits most wall frames.
• Skip it for small desk frames, borderless modern looks, and canvas or metal prints, which do not go under glass.

If the mat that came with your frame is damaged, discoloured or the wrong window size, we cut new ones to any opening as part of customised framing. Bring the frame or send us the measurements.