A4 paper format / International standard paper sizes (2025)

This explains the ISO 216 paper-size system built around an aspect ratio of √2:1 so that cutting or placing two sheets side-by-side yields the same proportions. Format A0 is defined to have an area of 1 m² and each An is half the area of the previous size; that gives A4 the standard dimensions 210 × 297 mm (A0 is 841 × 1189 mm). B-series sizes are the geometric mean between adjacent A sizes, and C-series sizes are the geometric mean between corresponding A and B formats (envelopes). Millimetre dimensions are derived by halving from n = 0 and flooring to the next lower integer millimetre so two A(n+1) never exceed An; tolerances are ±1.5 mm up to 150 mm, ±2 mm up to 600 mm, ±3 mm above. A noted exception is the Japanese JIS B series, which uses arithmetic means and different areas and is nonstandard internationally.

The system’s practical benefits are detailed with concrete examples: A4 is the everyday office size, A0/A1 for posters and technical drawings, A3 for diagrams, A5 for notepads, A6 for postcards, and C4/C5/C6 envelopes match unfolded/folded A4. Copier magnification factors (71%, 84%, 119%, 141%) map exactly between sizes, avoiding margin waste. Paper weight and postage calculations become trivial (80 g/m² means an A4 sheet ≈5 g, so 16 sheets ≈100 g). The format simplifies printing, repro, binding, shelving and is widely reused for IDs, passports and microfiche.