How ISO Paper Sizes Work (A, B and C)
ISO 216 paper sizes all share the same proportions (1 : √2). Cutting a sheet in half across its long side gives the next size down, so A4 is exactly half of A3.
The √2 ratio
Every A, B and C size has a long side that is about 1.414 times (√2) the short side. Because of this ratio, a sheet cut in half across its long side keeps the same shape, so scaling a document from A4 to A3 or A5 needs no change in layout. Enlarging A4 to A3 is about a 141% scale, and reducing A3 to A4 is about 70.7%.
A series
A0 has an area of one square metre (841 × 1189 mm). Each following size is half of the previous one: A1, A2, A3, A4 and so on down to A10.
B series
B sizes sit between A sizes. B0 is 1000 mm wide, and each B size is the geometric mean of the two nearest A sizes. B sizes are often used for books and posters. B7 is the size of a passport booklet.
C series
C sizes are for envelopes. A C4 envelope holds a flat A4 sheet, and a C5 envelope holds a flat A5 sheet or an A4 sheet folded once. The long DL envelope holds an A4 sheet folded in three.
Reading the names
The letter tells you the series and the number tells you how many times the base sheet has been halved. A larger number always means a smaller sheet. See how to read paper size names.
Frequently asked questions
Why is A4 210 × 297 mm?
A0 is defined as one square metre with a 1 : √2 ratio. Halving it four times gives A4, which rounds to 210 × 297 mm.
Why do larger numbers mean smaller paper?
Each step in the series halves the sheet, so A5 is half of A4 and A4 is half of A3.
Related
- A4 Paper Size210 × 297 mm
- A0 Paper Size841 × 1189 mm
- B4 Paper Size250 × 353 mm
- C4 Envelope Size229 × 324 mm
- How to Read Paper Size Names (A4, B5, C5, DL, ANSI)What the letters and numbers mean
Last updated: 2026-10-03