Paper Size Calculator (A0 to A6)
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/6/2026
A4, the most common size, measures exactly 210 × 297 mm under the international ISO 216 standard. Each size in the A series is obtained by folding the previous size in half lengthwise.
Explanation
The A series of paper sizes (A0 to A10) is defined by the international ISO 216 standard, used in almost the entire world with the notable exception of the United States and Canada, which use different sizes (Letter, Legal). Its remarkable property: each size is obtained by halving the larger dimension of the previous size, which gives a constant width-to-height ratio of 1:√2 (about 1:1.414) for every size in the series. This property guarantees that an A3 document photocopied at reduced scale onto A4 keeps exactly the same proportions, with no distortion or unnecessary white margins — one reason for this system's lasting popularity. The A0 size (841 × 1189 mm) has an exact area of one square meter, and serves as the reference from which every other size in the series is defined through successive halvings.
Example: the A4 size
Inputs
Size: A4.
Calculation
The A4 size measures 210 × 297 mm, exactly half of the A3 size (297 × 420 mm) folded lengthwise.
Result
The A4 size measures 210 mm wide by 297 mm tall.
Frequently asked questions
Why is the width-to-height ratio 1:√2 and not some other number?
Because it's the only ratio that stays identical after folding the sheet in half lengthwise: halving a sheet with a 1:√2 ratio gives a new sheet with the same 1:√2 ratio, simply smaller. This mathematical property is what lets the entire A series share the same proportions.
Does the United States use this system too?
No, the United States and Canada use different sizes, notably Letter (216 × 279 mm, slightly wider and shorter than A4) and Legal (216 × 356 mm), which don't follow the same constant-proportions principle as the ISO 216 series.
What is the difference between the A series and the B or C series of the ISO 216 standard?
The A series is the most used day to day (stationery, printing). The B series defines intermediate sizes slightly larger, used notably for posters and some passports. The C series, defined by a related standard (ISO 269), is used mainly for envelopes, sized to hold a corresponding A-size sheet without folding.