International frame sizes

How A-series, metric and US sizes relate

Three systems, three different sets of round numbers. The ISO A-series (A4, A3, A2…) is defined in millimeters and built on one rule: every size is exactly half the one above it, folded across the long edge, so they all share the same shape — an aspect ratio of 1:√2, about 1:1.41. A3 is two A4s; A2 is two A3s.

The metric (cm) frames are a separate, rounder grid — 30×40, 40×50, 50×70 — that doesn’t follow the A-series ratio at all. And the US inch sizes (8×10, 16×20, 18×24) are a third grid again, built on 4:5 and 2:3 shapes.

Here’s the catch that surprises everyone: no A-size and no round-cm size is exactly equal to a US frame. The nearest US frame is always a little bigger — sometimes a hair (a 40×50 cm canvas is ~⅛ inch under a 16×20), sometimes a couple of inches (A3 in a 16×20). That gap is why a mat or a slim spacer is almost always part of the answer — and it’s exactly what the “Nearest US frame” column above tells you.

One vocabulary note before you shop abroad: what Americans call a mat is a mount in the UK and Australia, and the opening an aperture. Same board, different word — the glossary’s dialect table has the full translation.

The traps that cost you a reprint

• A4 is not US Letter. A4 is 8.27×11.69 in; Letter is 8.5×11. A4 is narrower but noticeably taller, so an A4 print overhangs a Letter frame. Buy an A4 frame, or a US 9×12 with a small mat.
• A3 doesn’t fit a 12×16. At 11.69×16.54 in, A3 is just taller than a 12×16 frame’s 16 inches. Step up to a 16×20 (with a mat) or buy an A3 frame.
• “A2 ≈ 18×24” — but with a gap. A2 is 16.54×23.39 in, so it slips inside an 18×24 with about ¾ inch to spare on the sides. That’s a feature: the leftover becomes a clean mat border.
• A 40×50 cm canvas isn’t a perfect 16×20. 15.75×19.69 in sits about ⅛ inch proud of the frame on each side, so a diamond painting can slide around. Add a foam spacer or a slip mat — or buy a true 40×50 cm frame.
• Mixed-unit listings. A “40×50” frame might be centimeters (≈16×20 in) or inches. Check the unit before you order a print to match it.

Worked example: A4, A3 and the 1:√2 shape

Because every A-size halves into the next, they all share one shape — which is also why none of them line up with a 4:5 or 2:3 US frame. To size the mat that bridges the gap, set the frame size calculator to centimeters, or check the fit first with will it fit.

Framing a US print in Europe (inches → cm)

Going the other way is messier, because the cm grid is built on different round numbers than the inch grid — so a US print rarely matches a ready-made cm frame either. Order the frame first and print to its size when you can; otherwise budget for a mat. The closest common cm frame for the sizes people cross-shop:

• 8×10″ (20.3 × 25.4 cm) → nearest ready-made 30×40 cm frame, with a mat.
• 11×14″ (27.9 × 35.6 cm) → nearest ready-made 30×40 cm frame, with a mat.
• 16×20″ (40.6 × 50.8 cm) → nearest ready-made 50×60 cm frame, with a mat.
• 18×24″ (45.7 × 61.0 cm) → nearest ready-made 50×70 cm frame, with a mat.

Watch the 16×20: at 40.6×50.8 cm it’s a touch larger than a 40×50 cm frame, so it won’t squeeze in — size up to 50×60 cm, or have a 40×51 cm frame made.

Which frame should I actually buy?

• A-size print (A4/A3/A2): buy the matching A-size frame if you can find one — it’s the exact fit. No A-frame in stock? Use the nearest US frame from the chart and add a mat.
• Diamond painting / canvas in cm: a true cm frame (40×50, 50×70) is cleanest. A US 16×20 works for a 40×50 cm piece with a thin spacer or slip mat to stop it shifting.
• Oversize (A1, A0, 70×100 cm): these run past the biggest off-the-shelf US frame (24×36), so order a custom frame at the exact size.

One catch with diamond paintings and many kits sold by their cm size: the size on the box is the painted area, but the canvas runs larger, with a blank margin all around. Measure the part you actually want to show — not the full canvas — then size a mat to it in the mat border calculator.