Every number on this site can be traced to a published standard or to a formula in that standard. This page says which.
Generated, not typed
ISO 216 defines the A series from a single fact: A0 has an area of one square metre and sides in the ratio 1 to the square root of two. Every smaller size is the previous one halved along its long edge, rounded down to the millimetre. We generate the A, B and C series from those rules and check the result against the tables in ISO 216 and ISO 269. The same approach covers ISO 217 (RA and SRA sheets) and ANSI/ASME Y14.1 (ANSI and Architectural sizes).
Cross-checked
Sizes that come from trade practice rather than a formula, such as photo prints, US envelopes, book trims and newspaper formats, are checked against at least two independent published references before they are listed. Where a size genuinely varies between suppliers, the page says so in the dimension note rather than pretending to a precision that does not exist.
Units and rounding
- Millimetres are the stored value. Everything else is calculated from them.
- Inches are shown to two decimals and, where the traditional fraction is standard, as a fraction to the nearest sixteenth.
- Pixels are rounded to the nearest whole pixel at the stated DPI.
- Areas use the millimetre dimensions, so A4 shows 623.7 cm² rather than the nominal 625.
Standards referenced
- ISO 216:2007 (A and B series), ISO 217:2013 (RA and SRA), ISO 269:1985 (C series envelopes)
- ANSI/ASME Y14.1-2012 (ANSI and Arch sizes)
- JIS P 0138 (Japanese B series), GB/T 148-1997 (Chinese sizes), CAN 2-9.60M (Canadian P series)
- ISO/IEC 7810 (ID-1 cards), ICAO Doc 9303 (passport photos)
- AFNOR NF Q 02-000 (historical French sizes), British Standard 4000 and pre-metric trade tables (Imperial sizes)
Corrections
If you find an error, send the page link and the source you are checking against. Corrections are made within a few days and noted on the page.