A4 size in mm
A4 paper is exactly 210 × 297 mm. That is the ISO 216 definition — millimetres are how the standard is written, so these are the precise figures.
210 mmwidth
297 mmheight
ISO 216standard
Paper sizes to scale
Every outline is drawn to true relative scale. The dashed blue outline is US Letter.
The A series in millimetres
| Size | Millimetres |
|---|---|
| A0 | 841 × 1189 mm |
| A1 | 594 × 841 mm |
| A2 | 420 × 594 mm |
| A3 | 297 × 420 mm |
| A4 | 210 × 297 mm |
| A5 | 148 × 210 mm |
| A6 | 105 × 148 mm |
| A7 | 74 × 105 mm |
Why 210 × 297?
Every A size has the same aspect ratio, 1:√2 (about 1:1.414). Halving an A3 along its long edge gives exactly two A4 sheets with identical proportions. The series starts at A0, which is one square metre in area — from there each size is the previous one folded in half. That is why the numbers look arbitrary but the system is not.
The 1:√2 ratio is the reason scaling A4 to A3 on a photocopier fills the page perfectly with no cropping — a trick US Letter cannot do, because its proportions change from size to size.
Frequently asked questions
What is A4 size in mm?
Exactly 210 × 297 mm.
Is A4 exactly 210 mm wide?
Yes — ISO 216 defines A4 as 210 mm by 297 mm exactly, with a manufacturing tolerance of about ±1.5 mm.
What is A3 in mm?
297 × 420 mm — exactly twice the area of A4.