Paper Size Converter

One ratio, halved again and again

The A-series is built on a single idea: every size has the same shape, with sides in the ratio 1 to the square root of 2 (about 1:1.414), so cutting a sheet in half across its long side gives two sheets of the next size down with exactly the same proportions. That is why an A3 sheet photocopied at 71% fits an A4 sheet perfectly, and why scaling between A sizes never distorts anything. The series starts at A0, which has an area of exactly one square metre (841 × 1189 mm), and each step down halves the area.

A-series dimensions

Dimensions are exact by the ISO 216 standard, rounded to whole millimetres; inch figures are the same dimensions divided by 25.4 and rounded to two decimals.

A4 in practice

A4 is the default for letters, schoolwork and office printing across Europe, Asia, Africa and South America, and it is the size most home printers expect. Envelopes follow a parallel C-series: a C4 envelope (229 × 324 mm) takes an unfolded A4 sheet, a C5 takes A4 folded once, and a DL envelope (110 × 220 mm) takes A4 folded into thirds — which is why business letters arrive in DL envelopes. Notebooks, planners and flyers usually quote A5 or A6 when they are half or a quarter of A4.

North America is different

The United States and Canada use US Letter (8.5 × 11 inches, 216 × 279 mm) instead of A4: slightly wider and noticeably shorter. The difference is small enough to cause real annoyance — a document laid out for A4 can spill or shrink when printed on Letter, and vice versa. If you are sending a file to print abroad, set the page size explicitly rather than trusting the printer's default.