Paper sizes

Dimensions in millimetres, inches and pixels for every standard format, plus what each one is actually used for.

Comparing two sizes? Every pair close enough to be a real alternative, with the print scale that puts one on the other.

ISO A series

The international standard, used for everyday paper almost everywhere outside North America. Each size is the previous one cut in half across its longer side, and every size keeps the same 1:√2 proportions.

ISO B series

Sits geometrically between consecutive A sizes. Used where the A jump is too coarse — posters, books and envelopes.

ISO C series

Envelope sizes. C(n) is designed to hold A(n) unfolded, which is the whole point of the series.

North American

Letter, Legal and the ANSI drawing sizes. Defined in inches and not related to the ISO system, which is why A4 documents print badly on Letter and vice versa.

Cards

Business card standards, which differ between North America and Europe.

Photo prints

Common print and frame sizes, defined in inches.

Architectural

The US architectural series, defined in inches and built on a 4:3 or 3:2 ratio rather than the ANSI doubling. Drawing offices use ARCH where engineering offices use ANSI.

JIS B series (Japan)

The Japanese B series, which shares its names with ISO B and not its sizes. JIS defines B0 as 1.5 square metres against the A series’ 1, so every JIS B sheet is slightly larger than the ISO B of the same number — JIS B5 is 182 × 257 mm where ISO B5 is 176 × 250.

US envelope sizes

The American envelope standards: the commercial numbers, which carry folded Letter sheets, and the announcement A sizes, which carry cards. The announcement names collide with ISO A paper and have nothing to do with it — an A2 envelope is 111 × 146 mm where A2 paper is 420 × 594.

Book trim sizes

The finished page sizes commercial printers and print-on-demand services offer by name.

The two systems, and why they do not mix

Most of the world uses ISO 216 — the A, B and C series — which is built on a 1:√2 ratio so that halving a sheet preserves its proportions. North America uses Letter and Legal, defined in inches with no such property. A4 is 210 × 297 mm; Letter is 216 × 279 mm. Neither scaling nor rotating turns one into the other, which is why cross-Atlantic PDFs so often print with clipped edges.

Working out any A size without a table

The whole series comes from one definition: A0 is one square metre, in the 1:√2 proportion, and every size after it is the one before cut in half. So A5 has an area of 1/2⁵ of a square metre, which is 1/32 m² — and knowing that, the dimensions follow without looking anything up.

Two consequences are worth carrying around. The first is that A(n) is exactly half of A(n−1), so an A5 is half an A4 and a quarter of an A3, and an A5 is 1/32 of an A0 — which is how you get to A9 or A10 in your head from a number you already know. The second is the scaling factor: because the proportions never change, enlarging one step is always ×√2 and reducing one step is always ÷√2. Those are the 141% and 71% buttons on every photocopier ever made, and they are not approximations rounded for convenience — they are the ratio itself.

A quick check on any of it: an A4 sheet is 210 × 297 mm, and 210 × 297 × 16 is within a whisker of one square metre, because A4 is A0 halved four times.

What the B and C series are for

The A series jumps by a factor of two in area, which is a large step when a poster needs to be "a bit bigger than A2". B fills those gaps: each B size is the geometric mean of the A size with the same number and the one above it, so B4 sits between A4 and A3 rather than being a size of its own invention.

C is the envelope series, and it is built the same way one level in: C(n) is the geometric mean of A(n) and B(n), which makes it just larger than A(n). That is the entire point — a C4 envelope takes an unfolded A4 sheet, a C5 takes an A4 folded once, a C6 takes it folded twice. The relationship is why the numbers match rather than being a coincidence worth memorising.

DL, the long business envelope, is the exception: it is not part of the C series at all, and it exists because an A4 sheet folded in three is a shape the geometric series does not produce.

Choosing a resolution

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