Quick answer: ISO 216 is the international standard that defines the trimmed A and B series used for writing paper and many printed products. The system is built around the 1:√2 aspect ratio, allowing each sheet to be halved while preserving the same shape.
The familiar A4 office page is one member of a mathematically connected family. Instead of choosing unrelated dimensions for every format, ISO 216 creates a predictable scaling system. That consistency simplifies copying, enlarging, reducing, filing, folding and specifying print.
What ISO 216 covers
ISO 216:2007 specifies trimmed sizes for writing paper and certain classes of printed matter used for administrative, commercial and technical purposes. The standard itself notes that newspapers, books, posters and other special products may be governed by different standards or industry conventions. In other words, A sizes are widely useful, but they are not mandatory for every paper product.
The A-series principle
A0 is defined so that its nominal area is one square metre and its sides have the ratio 1:√2. Each subsequent size is formed by halving the previous sheet parallel to its shorter side. The resulting sheet has the same proportion after rotation.
| Size | Dimensions | Area relative to A0 |
|---|---|---|
| A0 | 841 × 1189 mm | 1 |
| A1 | 594 × 841 mm | 1/2 |
| A2 | 420 × 594 mm | 1/4 |
| A3 | 297 × 420 mm | 1/8 |
| A4 | 210 × 297 mm | 1/16 |
| A5 | 148 × 210 mm | 1/32 |
| A6 | 105 × 148 mm | 1/64 |
| A7 | 74 × 105 mm | 1/128 |
| A8 | 52 × 74 mm | 1/256 |
| A9 | 37 × 52 mm | 1/512 |
| A10 | 26 × 37 mm | 1/1024 |
This is why A3 is twice the area of A4, and A5 is half the area of A4. The published dimensions are rounded to whole millimetres, so multiplying the two rounded sides does not always produce the theoretical area exactly.
The B series
The B series uses the same aspect ratio but provides sizes that sit between neighbouring A formats. B0 is 1000 × 1414 mm. B sizes are useful for posters, books, artwork and production situations where an A size is either too small or unnecessarily large.
The complete dimensions and applications are set out in the B-series paper size guide.
Why the square-root-of-two ratio matters
A rectangle with sides in the ratio 1:√2 retains its proportion when divided into two equal halves across the long dimension. This property makes proportional copying straightforward. A page can move up one A size at 141.4% or down one size at 70.7%. In practice, copier menus often provide A4→A3 and A3→A4 presets that use this relationship.
ISO paper sizes and envelopes
C-series envelope dimensions were historically specified in ISO 269, which is now withdrawn. The familiar C4, C5 and C6 names remain widely used in stationery and envelope markets. A C4 envelope accepts an unfolded A4 sheet; C5 accepts A5 or A4 folded once; C6 accepts A6 or A4 folded twice. See the current practical dimensions in the C-series guide and verify them with the envelope supplier.
Trimmed sizes versus untrimmed sizes
ISO 216 concerns finished trimmed sizes. Commercial printers often buy or print on larger sheets so that images can bleed beyond the finished edge and the job can be gripped, registered and trimmed. ISO 217 defines RA and SRA untrimmed ranges for this production purpose. The {internal(“ra-and-sra-paper-sizes-explained”, “RA and SRA guide”)} explains the difference.
Countries and compatibility
ISO A sizes are used in most of the world. The United States and parts of North America commonly use Letter, Legal and related inch-based formats. Software and printers often support both systems, but page substitution can alter line breaks, margins and pagination. Use the actual target size from the beginning of a project rather than converting at the final export stage.
Frequently asked questions
Is ISO 216 the same as DIN 476?
DIN 476 was the influential German paper-size standard on which the international A-series system was based. Current international specifications should be referenced by their applicable ISO or national adoption.
Does ISO 216 define C envelopes?
No. ISO 216 defines A and B trimmed paper sizes. C envelopes were specified in ISO 269, a standard whose ISO record now shows it as withdrawn, although the size names remain widely used.
Can an A-series design be scaled with no changes?
The proportions remain correct, but typography, line weight, image detail and viewing distance still need review. A poster reduced to a leaflet can become illegible even though nothing is cropped.
Sources and standards
The measurements and production guidance in this guide were checked against the following standards bodies, postal operators and print-platform specifications. Standards can be revised or withdrawn, so check the linked record when a contractual or regulated specification matters.