Designing a document, a print layout, or just trying to remember whether A4 or Letter is bigger? The Paper Size Converter gives you the full dimension reference for every common paper size system, in whichever unit you prefer.
The logic behind the A-series
ISO 216’s A-series has an elegant mathematical property: each size is exactly half the area of the size before it, achieved by folding along the longer side, while every size maintains the same 1:√2 aspect ratio. That’s why A4 fits perfectly into A3 folded in half, and A3 into A2, all the way up to A0 (exactly 1 square meter).
Why US Letter doesn’t match A4
US Letter (8.5“ × 11“) follows a completely different historical standard than ISO 216, with a slightly different aspect ratio than A4. This is exactly why a document formatted for Letter doesn’t scale cleanly onto A4 paper and vice versa — something anyone who’s exported a US-formatted PDF for European printing has run into.
B-series and ANSI, too
Beyond A-series, the tool also covers B-series sizes (used for posters, passports, and some envelope formats) and US/ANSI engineering drawing sizes, which mirror the ISO doubling-area logic but starting from Letter-size ANSI A.
Try it yourself
Switch between A-series, B-series, and US/ANSI on the Paper Size Converter, and pick mm, cm, or inches to match whatever you’re designing for.