Last updated: 2026-06-28

Paper Sizes

A paper size converter returns the exact width and height of a standard sheet, so you can set up a document, choose an envelope, or scale artwork without guessing. It covers the ISO 216 A and B series used across most of the world — A4, A3, A5, B5 and the rest — alongside the US sizes Letter, Legal, Tabloid, and Executive. Dimensions are given in millimetres and inches.

The ISO 216 system

The international standard ISO 216 defines the A and B series. Every size in a series shares one aspect ratio: the height divided by the width equals $\sqrt{2}$, about 1.414. That single rule gives the system its defining property — cutting a sheet in half across its longer side yields two sheets of the next size down, each with the same proportions.

Because of this, $A5$ is exactly half of $A4$, $A4$ is half of $A3$, and so on up to $A0$, which has an area of one square metre. Doubling or halving a layout therefore keeps margins and images in proportion, which is why "fit to A4" scaling in a print dialog never distorts the page.

The B series sits between the A sizes. A $Bn$ sheet is the geometric mean of $An$ and $A(n-1)$, making B formats useful for posters, books, and envelopes that must hold an A-size sheet.

US sizes

North America uses a separate set of sizes defined in inches rather than by a constant ratio. US Letter is 8.5 × 11 in (215.9 × 279.4 mm), Legal is 8.5 × 14 in, and Tabloid is 11 × 17 in. None of them share the $\sqrt{2}$ ratio, so a document laid out for Letter will not map cleanly onto A4 — the width and height both differ by several millimetres.

Worked example

Suppose you have an A3 poster and want to know whether it will fit through an A4 scanner at half size. A3 measures 297 × 420 mm. Halving it across the long side gives:

The result, 210 × 297 mm, is exactly A4 — confirming the halving property and that the reduced poster fits without cropping.

For unit-by-unit conversions outside these standard formats, the Length Converter handles arbitrary millimetre-to-inch values, and the Area Converter covers sheet areas.