Paper sizes
A0 to A6, Letter and Legal.

The A series from A0 to A6

ISO 216 starts from A0, a sheet of one square metre with sides in the ratio 1:√2. Each next size halves the long side, so the shape never changes and the area halves every step.

A0 is 841 × 1189 mm (33.11 × 46.81 in), A1 is 594 × 841 mm (23.39 × 33.11 in) and A2 is 420 × 594 mm (16.54 × 23.39 in): the poster and plan sizes.

A3 is 297 × 420 mm (11.69 × 16.54 in), A4 is 210 × 297 mm (8.27 × 11.69 in), A5 is 148 × 210 mm (5.83 × 8.27 in) and A6 is 105 × 148 mm (4.13 × 5.83 in): the office, flyer and card sizes.

Because the ratio holds, a layout scales from one A size to another without cropping. An A4 page enlarged to 141% (√2) fills A3.

Paper sizes in pixels

A sheet of paper has no fixed size in pixels: the number depends on the resolution you print at. Divide the millimetres by 25.4 to get inches, then multiply by the DPI.

At 300 DPI, the usual target for print, A3 is 3508 × 4961 px, A4 is 2480 × 3508 px, A5 is 1748 × 2480 px and A6 is 1240 × 1748 px. US Letter is 2550 × 3300 px.

Large posters seen from a distance are often designed at 150 DPI. A2 is then 2480 × 3508 px, A1 is 3508 × 4967 px and A0 is 4967 × 7022 px.

US Letter, Legal and Tabloid

Offices in the United States use inch-based ANSI sizes instead of the A series. Letter is 8.5 × 11 in (215.9 × 279.4 mm), the everyday sheet, and Legal is 8.5 × 14 in (215.9 × 355.6 mm), the same width and 3 in longer.

Tabloid is 11 × 17 in (279.4 × 431.8 mm), two Letter sheets side by side. Turned to landscape, 17 × 11 in, it is often called Ledger. At 300 DPI, Tabloid is 3300 × 5100 px.

Letter and A4 look alike but are not interchangeable: Letter is about 6 mm wider and 18 mm shorter, so a page made for one gets extra margins or a cut edge on the other.