Convert between every standard paper size — ISO A0–A10, B-series, C-series, US Letter / Legal / Tabloid and Japanese JIS — in mm and inches.
Built and maintained by Paul Clark, Redmoon Software · Checked against what this tool computes on
| Size | mm | cm | inches | Area (cm²) |
|---|
ISO A-series starts at A0 = 841 × 1189 mm (1 m² area, √2 aspect). Each next size halves the longer dimension. B-series is the geometric mean of consecutive A sizes, and C-series fits A inside (used for envelopes).
The scale ratio is min(B.w/A.w, B.h/A.h) — the largest factor that fits A inside B without cropping. 1 inch = 25.4 mm.
Two dropdowns, a fifty-entry lookup table and one ratio. The list holds ISO A0–A10, ISO B0–B10, ISO C0–C10, JIS B0–B10 and six US sizes — Letter, Legal, Tabloid, Executive, Statement and Ledger — all stored in millimetres. Scale A → B is min(B.width ÷ A.width, B.height ÷ A.height) × 100, the largest uniform reduction that fits A entirely inside B. A4 onto US Letter comes out at 94.1%.
Order matters, because A is the source and B is the destination; swapping the two gives a different percentage. The table underneath prints both sizes in millimetres, centimetres, inches at 25.4 mm to the inch, and area in square centimetres, which is the quickest way to see A-series areas halving step by step down from A0's one square metre. The defaults are A4 against US Letter, the comparison most people arrive for.
The percentage is a geometric fit, not a print setting. It ignores your printer's unprintable margin, so scaling A4 artwork to exactly 94.1% on Letter can still clip at the edges — most drivers quietly apply their own shrink on top. The dimensions are the published whole-millimetre values, themselves rounded from the underlying √2 geometry, and every size is listed portrait except US Ledger, stored as 431.8 × 279.4 mm landscape.
94.1%, which is what the tool returns with A4 as size A and US Letter as size B. Going the other way, Letter onto A4, gives 97.3%. Both are fit-to-page factors that keep the entire page visible without cropping anything.
A4 is narrower and taller: 210 × 297 mm against 215.9 × 279.4 mm. By area A4 wins, 623.7 cm² to 603.2 cm². Because the mismatch runs in both directions, neither prints on the other without some scaling.
They are not the same sheet. ISO B5 is 176 × 250 mm, defined as the geometric mean of A4 and A5. JIS B5 is 182 × 257 mm, derived from a larger area rule used in Japan. Both appear in the dropdown.
C4 at 229 × 324 mm takes an unfolded A4 sheet, C5 at 162 × 229 mm takes A4 folded once, and C6 at 114 × 162 mm takes it folded twice. The whole C-series is dimensioned to swallow its matching A size.
A0 is 841 × 1189 mm, one square metre at a √2 aspect ratio. Halving the long side gives each next size down, all the way to A10 at 26 × 37 mm, and the ratio is preserved so scaling between them never distorts proportions.
ISO 216 defines the A series with an aspect ratio of √2:1, and that ratio is preserved when a sheet is halved across its long side. A4 folded gives two A5s of the same proportion, which is why scaling artwork between A sizes needs no reframing — it is a pure percentage scale, 71% down or 141% up.
The series is anchored by A0 having an area of one square metre, which is what makes paper weight in grams per square metre directly meaningful.
US Letter, Legal and Tabloid are not part of that system and have no consistent aspect ratio between them. Letter is roughly 1.29:1 against A4's 1.41:1, so converting between them always either crops or leaves uneven margins — there is no scale factor that maps one to the other.
A4 is slightly narrower and taller than Letter, which is why a document laid out for one and printed on the other typically loses content at an edge or reflows unexpectedly.
For anything crossing regions, designing with generous margins and avoiding content near the trim is more robust than any conversion.
The standard or formula behind this tool, so you can check it rather than take it on trust.
Defines the A, B and C series. Every size is exactly half the previous one and the √2 aspect ratio survives every halving, which is why A4 artwork scales to A3 without reframing and to US Letter never does.
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