How Wide Should a Mat Be? One Framer's Own Pages Give Four Different Answers

That spread is not academic. Border width sets the size of the mat, the glass and the frame you pay for. Every centimetre of border adds two centimetres to the width and two to the height. Take the same poster from the narrowest published rule to the widest and the glazed area you are buying slightly more than doubles.

So pick a width on purpose, check the final outer size before you order, and ignore anyone who calls their number the professional standard.

Why does mat width decide what you pay?

A mat sits between the print and the frame, so the frame has to be big enough for the print plus the border on all four sides. The arithmetic is simple and it is the only piece of arithmetic in this guide you have to remember.

Add a border of 5 cm and the mat grows 10 cm wider and 10 cm taller. Add 13 cm and it grows 26 cm in both directions. A print that fits a shelf frame at one border width needs a custom frame at another.

This also catches people out in the other direction. IKEA states plainly that its frame sizes refer to the inner dimensions, “the space where your image or mount fits”, and that the external dimensions are larger and vary by design. Its own pairings show what that means once a mount is involved:

Those borders are our own arithmetic from IKEA’s two columns, half the difference in each case. Read down the right-hand column and you will notice IKEA does not scale the border with the art. It stays near 5 cm from the 40 x 50 cm frame upward, which already contradicts every proportional rule below.

What is the one-third rule, and who says it is the standard?

The most widely repeated rule is that the border should be at least one-third of the artwork’s shortest side. CustomPictureFrames.com, a US online custom framer, states it in the strongest possible terms on its border width proportions page:

The rule of thirds represents the fundamental professional standard for calculating minimum mat border width: borders should equal at least one-third of the artwork’s smallest dimension. This formula ensures proportional relationships that work universally across different artwork sizes.

“Universally” is doing a lot of work there. The same page then adds a hard floor that has nothing to do with proportion, saying borders “should never drop below 4 inches regardless of artwork size”, which is 10.16 cm. It also carves out an exception at the small end, where a 10.16 x 15.24 cm piece gets about 3.38 cm by thirds but the page says many framers prefer 5.08 to 6.35 cm, closer to half the short side than a third.

No study, author or date appears anywhere on that page to support the standard it names. That is an observation about the page, not a claim that the rule is wrong.

What do the same framer’s other three pages say?

This is where it gets genuinely difficult for a buyer. All four pages below were read on 10 October 2026, and all four belong to the same company.

Its how to measure page opens with the same rule, calling one-third “a good starting point” that “works well for most artwork sizes”. Then, a few hundred words later, it contradicts the rule for large work:

Large Artwork (over 24x36 inches): Use 3.5 to 4 inch borders. The one-third rule creates borders that are too wide at this size. A 30x40 print at one-third would need 10-inch borders, making a 50x60 inch frame.

Translated: for prints above 60.96 x 91.44 cm, use 8.89 to 10.16 cm, because one-third is too wide. The worked example says a 76.2 x 101.6 cm print would need 25.4 cm borders and a 127 x 152.4 cm frame.

Its dated blog post, last modified 29 April 2026, drops the fraction entirely for a percentage:

A reliable starting formula is to set the border width at roughly 15 to 20 percent of the shorter dimension of the artwork.

One-third is 33.3%. So the dated page recommends a little over half of what the undated page calls the fundamental professional standard.

And its picture matting guide sorts borders by look rather than by size, and sends large posters to the narrowest option of all:

1 to 1.5 inches, narrow, modern, gallery-style. Best for large posters and contemporary art prints.

That is 2.54 to 3.81 cm, for exactly the category the first page says should never go below 10.16 cm.

Four pages, four rules, one company. None of them references the others.

What do other framers publish?

Widening the sample does not produce a consensus, it produces a third kind of rule.

Metropolitan Picture Framing’s mat calculations page sets a floor and a band rather than a proportion:

We seldom recommend mat borders less than 2”. Sizes ranging from 16 x 20 and up generally get 2 1/2” to 5” borders. Traditionally, mat borders are equal on the top and sides and 1/2” to 1” extra on the bottom border.

In centimetres: rarely under 5.08 cm, and 6.35 to 12.7 cm for anything from 40.64 x 50.8 cm upward, with 12.7 to 25.4 mm extra along the bottom. Note that 6.35 to 12.7 cm is a two-fold range for one size band, which is an honest way of saying the choice is yours.

So the published rules come in three incompatible shapes. Some set the border as a share of the artwork, which scales. Some set a fixed band or floor in centimetres, which does not. And IKEA, in effect, sets it at about 5 cm and leaves it there. A proportional rule and a fixed-floor rule cannot both be right across sizes, because they diverge the moment the print gets large.

What does each rule do to a 40 x 50 cm print?

Here is every published rule above applied to one of the most common printable sizes. Short side is 40 cm.

The arithmetic in the right-hand column is ours: artwork plus twice the border, in both directions. The gap between the top row and the bottom row is about 16 cm of width, which is the difference between a frame you can buy off a shelf and one you cannot.

What does each rule do to a 61 x 91 cm poster?

This is where the four answers stop being a matter of taste. Short side is 61 cm.

The narrowest published answer gives a 66.1 x 96.1 cm sheet of glass. The widest gives 101.7 x 131.7 cm. By our arithmetic that is about 6,350 cm2 against about 13,400 cm2, so the widest rule asks you to buy slightly more than twice the glazed area of the narrowest. Both numbers come from the same framer’s website.

How much of your print does the mat actually hide?

Every mat overlaps the edge of the print, because the window has to be slightly smaller than the paper or the print falls through. That overlap is published too, and it also varies.

Metropolitan says to measure in “at least 1/8” from each edge (on larger pictures we recommend a minimum 1/4”)”, which is at least 3.175 mm, rising to 6.35 mm on larger pieces.

The practical consequence for a downloadable print is that you should not put anything you care about within about 6 mm of the edge of the paper, and you should not expect a visible border of your own design to survive matting. If your file has a thin white margin built in, the mat window will eat part of it and the result will look narrower on one side than another if the print is hung slightly off-centre in the window.

So how should you choose?

Four steps, and none of them need a rule.

Start from the frame you can actually buy. If a 50 x 70 cm frame is the one you own or the one that is cheap near you, work backwards: that frame minus your print, divided by two, is your border. A 40 x 50 cm print in a 50 x 70 cm frame gives 5 cm on the sides and 10 cm top and bottom, which is a real and common look.

Then decide what the border is for. A wide border makes a small print feel deliberate and formal. A narrow one reads as modern and gallery-like, which is exactly why the matting guide recommends it for posters. Neither is incorrect.

Then weight the bottom only if you want to. Metropolitan calls 12.7 to 25.4 mm of extra depth along the bottom traditional, and the same page notes that many people keep all four borders equal so the mat can be reused on a horizontal or a vertical picture. That is a better argument than tradition.

Finally, and this is the one that costs money: read the final outer dimensions on the order screen before you pay, not the artwork size you typed in. On a made-to-measure framer, adding a mat enlarges the frame, because the frame is built around the art plus the border. On a retail frame the label is the opening, so adding a mount means the artwork inside has to get smaller instead.

Sources and limits

Every page quoted here was fetched and read on 10 October 2026, and every quotation was matched against the live page text. Three limits worth stating.

The four CustomPictureFrames.com pages are not all dated. The blog post carries a modified date of 29 April 2026; the three resource pages show no editorial date at all, and the site’s sitemap gives one shared regeneration timestamp to almost every URL, so it cannot be used to tell which page is newer. We therefore cannot say which of its four rules is its current position.

Every centimetre figure here is converted from a published inch figure at 2.54 cm to the inch, except the IKEA numbers, which IKEA publishes in centimetres. Conversions are shown so you can check them.

And we did not find a trade-body standard for mat width in this search, which is not the same as establishing that none exists.

Pin this guide

Save any of these to your Pinterest boards. They link straight back to this guide.

• 

•