The European Standard: Frame Sizes, Ratios, and Matting for Fine Art

A frame size is not the same thing as an image size. That single distinction resolves most framing mistakes.

The print has its own dimensions. A mount, sometimes called a mat or passe-partout, adds space around it. The frame holds the combined result. Before buying anything, confirm which of those three measurements a listing describes.

The short answer

For an unmatted A3 print, look for a frame made for 29.7 × 42 cm artwork. For an unmatted A2 print, look for 42 × 59.4 cm artwork. If you want a mount, the outer frame must be larger than the print. A 50 × 70 cm frame can hold many smaller prints, but only with a mount cut to the image.

European A-series paper follows ISO 216. Its defining proportion is 1:√2, so each size can be halved and keep the same shape. A3 and A2 therefore belong naturally together, but neither has exactly the same ratio as 50 × 70 cm.

| Print or frame name | Exact dimensions | Approximate inches | Ratio | | --- | ---: | ---: | ---: | | A4 | 21 × 29.7 cm | 8.3 × 11.7 in | 1:1.414 | | A3 | 29.7 × 42 cm | 11.7 × 16.5 in | 1:1.414 | | A2 | 42 × 59.4 cm | 16.5 × 23.4 in | 1:1.414 | | 50 × 70 cm | 50 × 70 cm | 19.7 × 27.6 in | 1:1.4 |

The difference between 1:1.414 and 1:1.4 is small, but real. An A2 sheet placed directly into a 50 × 70 cm opening leaves roughly 8 cm horizontally and 10.6 cm vertically before allowing for the frame rebate. It should be treated as a mounted presentation, not as an exact fit.

Measure what the frame accepts

Frame retailers use “size” inconsistently. It may mean the outside edge of the moulding, the glass size, or the artwork the frame is intended to accept. The useful measurement is the opening behind the frame rebate: the space available for the print, mount, glazing, and backing.

Ask four questions:

1. What artwork size does the frame accept without a mount?
2. What is the visible opening of the supplied mount?
3. What are the external frame dimensions?
4. How deep is the rebate?

The visible opening is normally cut slightly smaller than the image so the paper cannot fall through. This means a print labelled 30 × 40 cm should not automatically be paired with a window opening of exactly 30 × 40 cm. Confirm the actual image area and leave enough overlap to hold the sheet safely.

Standard frame or custom frame?

A standard, off-the-shelf frame is sensible when its opening matches the sheet or when a mount can reconcile the difference cleanly. It is easier to replace, usually less expensive, and lets a group of works share a quiet visual rhythm.

Custom framing becomes useful when:

• the image has an uncommon ratio;
• the paper edge should remain visible;
• the work needs a deliberately wide or asymmetric mount;
• the glazing, backing, depth, or finish needs to meet a particular conservation brief;
• a standard frame would force an awkward crop.

Do not crop a photograph merely to satisfy a frame unless the work is explicitly offered for cropping. The composition belongs to the photograph; the frame should accommodate it.

What a mount changes

A mount is both visual space and physical separation. It can keep the print from touching the glazing, create a calmer transition between image and wall, and adapt one image size to a standard frame.

There is no universal correct width. A narrow mount feels compact and graphic. A wider mount gives the image more breathing room and can make a modest print hold a larger wall. Many framers make the lower border fractionally deeper than the top and sides; this is optical balancing, not a requirement.

For preservation-minded framing, ask for materials suitable for photographs and works on paper. ISO 18902 covers physical and chemical requirements for albums, storage, and framing materials used with processed imaging materials. A useful conversation with a framer is more specific than the word “archival”: ask what the mount board, hinges, backing, and glazing are, and whether the assembly is reversible.

Metric and imperial are not interchangeable

Imperial sizes can look close to metric ones while remaining different enough to cause trouble.

| Imperial size | Metric equivalent | Nearby metric size | | --- | ---: | ---: | | 8 × 10 in | 20.3 × 25.4 cm | 20 × 25 cm | | 11 × 14 in | 27.9 × 35.6 cm | 30 × 40 cm | | 16 × 20 in | 40.6 × 50.8 cm | 40 × 50 cm | | 20 × 28 in | 50.8 × 71.1 cm | 50 × 70 cm |

“Nearly” is not enough for direct framing. A few millimetres can buckle paper or leave an unwanted gap. Convert the stated dimensions, then compare the exact opening.

A dependable ordering sequence

1. Confirm the sheet dimensions and the image dimensions.
2. Decide whether the paper edge should be hidden, overlapped, or shown.
3. Choose direct framing or a mount.
4. Confirm the frame opening and rebate depth.
5. Choose glazing and backing for the room and the work.
6. Only then choose the moulding finish.

The quietest result often comes from treating the frame as architecture: proportion first, finish second.

Sources and further reading

• ISO 216:2007 — trimmed sizes for A and B paper series
• ISO 18902:2013 — albums, framing, and storage materials for processed imaging materials

About this guide: prepared as practical orientation, not a substitute for advice from a qualified conservator or framer who can inspect the specific print and room.