Data & Converters
World Paper Size & Dimension Converter
Convert any length between millimetres, centimetres and inches, and look up standard paper sizes (A-series, US Letter, Legal) used around the world.
Dimension converter
mm
210.0
cm
21.00
inches
8.27
Standard paper sizes (mm)
The world's two paper systems
Paper comes in two main systems: the A-series (ISO 216), used almost everywhere, and the US Letter/Legal/Tabloid family, used in North America. A4 (210 × 297 mm) is the office default across Europe, Asia, Africa and Oceania; Letter (216 × 279 mm) is the default in the US and Canada. The two are close but not interchangeable, which trips up anyone designing documents for both.
This tool converts any length between millimetres, centimetres and inches, and lists the standard sizes in each system, so you can design for either and translate dimensions precisely.
The mathematical beauty of the A-series
The A-series is defined by a single property: every size has an aspect ratio of √2 (about 1.414). This means folding an A-series sheet in half along its long side produces the next smaller size with exactly the same ratio — A3 folds to A4, A4 to A5, and so on. A0 is defined as one square metre, and every smaller size is a power-of-two fraction of it.
This elegant property makes the A-series scale cleanly: a brochure designed on A5 prints identically on A4 (two per sheet) or A3 (four per sheet) with no reflow. It is one of the few standards where the mathematics genuinely helps the user.
US Letter and its family
The US system — Letter, Legal and Tabloid — predates ISO 216 and has no mathematical relationship between sizes. Letter is 8.5 × 11 inches, Legal is 8.5 × 14 (the same width, taller, for contracts), and Tabloid is 11 × 17 (two Letters side by side). The sizes derive from historical paper-making dimensions rather than a ratio.
Because Letter is slightly wider and shorter than A4, a document designed to fit A4 may reflow or clip on Letter, and vice versa. When designing for both, leave margins or use a size that works on both; PDFs set to A4 printed on Letter paper often cut off the bottom.
Converting dimensions precisely
The millimetre is the base for the A-series; the inch is the base for the US system. The conversion is exact: 1 inch = 25.4 mm by definition. So A4's 297 mm is 11.69 inches, and Letter's 11 inches is 279.4 mm. This tool converts any dimension between mm, cm and inches using that exact ratio.
For design work, convert once and work in a single unit to avoid accumulated rounding. Switching back and forth between mm and inches in a layout tends to introduce small errors that compound; pick the unit your printer or manufacturer uses and stay in it.
Choosing the right size for printing
If you print in a region, use that region's default. Designing a flyer for Europe? A5 or A4. For North America? Letter or half-Letter. Sending a digital document meant to be printed anywhere? Many people design to A4 and accept that North American printers will scale or clip slightly — or design with generous margins that work on both.
For booklets and brochures, the A-series' folding property is a real advantage: an A4 sheet folded twice becomes an A6 booklet with no waste. The US equivalent (Letter folded to a quarter) leaves an uneven flap, which is why even some North American designers prefer A-series for folded work.
Beyond office paper
The A-series extends in both directions: larger A0, A1 and A2 are used for posters and technical drawings; smaller A6 and A7 for cards and tickets. The related B-series fills gaps between A sizes (B5 is common for books), and the C-series defines envelopes (C4 fits A4, C5 fits A5). US envelopes follow a different naming. This tool focuses on the common sizes; the full ISO 216 family follows the same √2 ratio throughout.
Knowing the system your document or product targets prevents costly print errors. A few minutes checking sizes before sending a file to print — especially across the A/US divide — saves reprints and keeps layouts clean.
Frequently Asked Questions
What is the A-series?
The A-series (ISO 216) is the paper standard used almost everywhere except North America. A0 is one square metre; each smaller size halves the area, so A4 is half of A3. The aspect ratio is √2, which preserves the shape when folded.
What is US Letter size?
US Letter is 8.5 × 11 inches (216 × 279 mm), the standard office paper in the US and Canada. It is slightly wider and shorter than A4, which causes formatting differences when documents cross between the two systems.
Why do A4 and Letter differ?
Different standards history: the A-series was defined by a German standard in the 1920s on a mathematical ratio; US Letter descends from older loose-sheet sizes. Neither is 'better', but documents designed for one do not always print cleanly on the other.
How do I convert mm to inches?
Divide millimetres by 25.4 (1 inch = 25.4 mm exactly). To go from inches to mm, multiply by 25.4. This tool does both instantly and to centimetres as well.
What about Legal and Tabloid?
US Legal is 8.5 × 14 inches, used for contracts. Tabloid (11 × 17 inches) is twice Letter area, used for larger layouts and booklets. Both are common in North American offices.
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