Paper Size Reference
ISO A/B-series and US paper sizes in millimeters and inches.
Overview
The Paper Size Reference lists the ISO A-series and B-series papers along with the common US sizes — Letter, Legal, Tabloid, Executive — in both millimetres and inches. Whether you're setting up a print job, choosing a frame for a poster or porting a US-letter PDF to A4, the reference has the dimensions you need.
It is used by designers preparing print-ready layouts, office workers handling international correspondence and print-shop staff matching paper stock to a customer's requirements. Long-tail queries it answers include "A4 dimensions in inches", "B5 paper size mm", "Letter vs A4 dimensions" and "ISO 216 paper size table".
How it works
ISO 216 defines the A-series and B-series so each smaller size is exactly half the area of the next larger one, with the same aspect ratio of 1 to √2. A0 has an area of exactly 1 square metre (841 × 1189 mm); A1 is A0 folded in half; A4 is A0 folded four times. The B-series fills the gaps between A-series sizes, with B0 measuring exactly 1 metre on its short edge.
US sizes are imperial fractions inherited from older standards: Letter is 8.5 × 11 inches, Legal is 8.5 × 14 inches, Tabloid is 11 × 17 inches. The reference shows each US size in both inches and millimetres so it can be matched against ISO sizes.
Examples
A4 → 210 × 297 mm → 8.27 × 11.69 in
A3 → 297 × 420 mm → 11.69 × 16.54 in
B5 → 176 × 250 mm → 6.93 × 9.84 in
US Letter → 216 × 279 mm → 8.5 × 11 in
FAQ
Why does folding an A4 in half give an A5?
ISO 216 chose the aspect ratio 1 to √2 specifically because folding the long edge in half preserves the same aspect ratio. The new piece is the next size down, and the height/width relationship doesn't change.
Is US Letter close enough to A4 to swap?
They are similar but not interchangeable. A4 is narrower and longer than Letter. A document designed for one will not fit perfectly on the other and usually needs to be re-laid-out or scaled at the print driver.
What's the difference between A-series and B-series?
A-series sizes are powers-of-2 divisions of 1 m². B-series sizes have heights that are the geometric mean of adjacent A-series sizes, filling in halfway between them. B5, for example, sits between A4 and A5.
Why doesn't the US use A4?
Historical convention. The US adopted 8.5 × 11 as the office standard in the 1980s based on existing paper-making and folder formats. Canada and Mexico are the main other countries that follow US sizes.
Are there sizes smaller than A10?
ISO 216 defines down to A10 (26 × 37 mm), which is roughly the size of a postage stamp. Smaller sizes are non-standard but follow the same √2 pattern by halving.