The ISO 216 standard defines the A and B paper series used almost everywhere outside North America. This reference lists every size from the wall-sized A0 down to the tiny A10, plus the full B series, in millimetres, centimetres, and inches.
How it works
ISO paper is built on one elegant rule: every sheet has the aspect ratio 1 : √2 ≈ 1 : 1.414. That ratio is the only one where folding a sheet in half across its long edge gives a smaller sheet of exactly the same shape. So two A5 sheets make an A4, two A4 sheets make an A3, and so on with no wasted paper and no distortion when you scale a document up or down.
The series is anchored by A0 = 841 × 1189 mm, whose area is almost exactly one square metre. Each next size halves the long dimension and rounds down to the whole millimetre, which is why A4 is 210 × 297 rather than a tidy fraction. The B series starts at B0 = 1000 × 1414 mm and fills the gaps between A sizes, so a B-size envelope comfortably holds the corresponding A-size sheet.
Example and notes
To print an A3 document onto two A4 sheets, you tile it — because A3 is exactly two A4s side by side, the halves line up perfectly. To shrink an A4 page to A5, scale to about 71% (which is 1 divided by √2): one A4 maps onto one A5 with no cropping.
Unit conversions in the table use 1 inch = 25.4 mm. Because the metric values are the official definition, the inch figures are rounded approximations — useful for setup, but specify sizes in millimetres for exact print work.