Metric frame sizes and their inch equivalents

Two frame systems exist in the world and they do not line up. Most people meet this the day they buy an art print from a European seller and discover that no frame in their local shop will hold it.

The two systems

Inches, used in North America, with sizes inherited from photographic print standards and US paper: 8x10, 11x14, 16x20, 8.5x11, 24x36.

Metric, used almost everywhere else, split into two families. The A-series — A5 through A1 — is a defined paper standard where every size has the ratio √2:1 and halving one gives exactly two of the next. The round centimetre sizes — 30x40, 40x50, 50x70 — are conventional retail sizes with no unifying ratio.

Neither system is better. They are simply incompatible, and the incompatibilities are frequently small enough to look like they might work, which is worse than being obviously different.

A-series sizes

Every one of these has a ratio of 1:1.414. No US frame size does. That single fact is the root of every A-size framing problem.

Round centimetre sizes

Notice that these ratios are actually closer to US frame ratios than the A-series is. 20x30 cm is 3:2. 24x30 and 40x50 cm are 4:5. 30x40 cm is 3:4. The round centimetre sizes are, in ratio terms, the same family of shapes as US frames — just at different absolute sizes.

The near-miss table

This is the part that costs people money. These pairs look interchangeable and are not.

Two rows deserve highlighting.

A4 in an 8.5x11 frame does not fit. The sheet is seven tenths of an inch taller than the opening. This is not a loose fit; it is an impossible one.

A 70x100 cm print in a 27x40 frame does not fit either. The print is wider than the frame's opening.

Everything else in that table fits with a gap. Gaps under about a quarter inch are tolerable — the print may shift slightly, and a strip of archival tape at the back holds it. Gaps over half an inch look like a mistake and need a mat.

The solution, in almost every case: a mat

When a metric print and an imperial frame disagree, the cheapest and best-looking fix is a custom mat cut to the print's actual dimensions.

A custom mat costs perhaps fifteen to forty dollars depending on size and board quality, arrives in a few days, and produces a result that looks entirely deliberate. A custom frame costs three to five times that.

So: buy the next standard frame up, and have a mat cut. An A3 print goes into an 18x24 frame with an A3 opening and looks like a gallery presentation. The same print forced into a 12x18 frame looks like a compromise, because it is one.

Converting properly

Multiply inches by 2.54 for centimetres; divide centimetres by 2.54 for inches. The important discipline is to round only at the end.

Converting 50 x 70 cm by rounding each dimension to the nearest inch gives 20 x 28. That happens to be right here. Converting 42 x 59.4 cm the same way gives 17 x 23, when the true figures are 16.54 x 23.39 — and a frame ordered on 17 x 23 is half an inch wrong in one direction.

If you would rather not do it by hand, the aspect ratio calculator converts between the two units, keeps the ratio locked, and lists which US frames match the shape.

Buying metric frames in the US

Some retailers stock A4 and A3 frames, particularly those selling European-designed furniture and homewares. Larger metric sizes almost always need an online order from a specialist.

Two things to check before ordering:

Whether the listed size is the opening or the outside. European listings occasionally give the outside dimension, which is a different number entirely.

Whether a mount is included. Many European frames ship with a mount — the British term for a mat — already cut, often to a smaller A size. An A3 frame with an A4 mount is a common and useful configuration, and it is not always obvious from the listing.

British and European terminology

If you are buying from a UK or European retailer, some words differ and the differences matter.

Mount means mat. A frame listed as "A3 with A4 mount" is an A3 frame containing a mat cut with an A4 opening — which is a genuinely useful configuration and not always obvious from the listing.

Mount board is mat board. Mount cutter is a mat cutter.

Glazing is used the same way in both systems, but "perspex" is the common British word for acrylic sheet.

Frame size in European listings usually means the artwork opening, as it does in the US — but not always. Some listings give the outside dimension of the moulding. Where a listing gives both, the smaller number is the opening.