A4 paper is the most widely used paper size in the world, measuring 210 × 297 millimeters (8.27 × 11.69 inches). It is part of the ISO 216 international standard, itself based on the German DIN 476 standard of 1922.
The Genius of the √2 Aspect Ratio
The defining feature of A4 — and the entire A series — is its aspect ratio of 1:√2 (approximately 1:1.4142). This means that when you cut an A0 sheet in half, you get two A1 sheets with the exact same proportions. Cut an A1 in half and you get two A2s, and so on down to A10.
This property is what makes A4 so practical. Documents can be scaled up or down without distortion. A poster can be reduced to a postcard, or a business card can be enlarged to a wall chart, all without cropping or stretching.
The Full A Series
- A0 — 841 × 1189 mm — one square metre
- A1 — 594 × 841 mm
- A2 — 420 × 594 mm
- A3 — 297 × 420 mm
- A4 — 210 × 297 mm
- A5 — 148 × 210 mm
- A6 — 105 × 148 mm
- A7 — 74 × 105 mm
- A8 — 52 × 74 mm
- A9 — 37 × 52 mm
- A10 — 26 × 37 mm
Why A4 Became Universal
Before ISO 216, almost every country had its own paper sizes. Britain had Foolscap and Quarto. The United States had Letter, Legal, and Tabloid. France had Raisin, Soleil, and Aigle. This chaos made international correspondence, publishing, and trade unnecessarily difficult.
ISO 216 provided a single, simple framework. A4 was the natural default: it fits into briefcases and folders, holds about 2,000 words of typed text, and folds neatly into a C5 envelope.
Common Uses of A4
- Office documents, letters, and contracts
- Academic essays, theses, and dissertations
- Magazines and brochures (folded)
- Standard printer paper worldwide
- Photocopying and scanning
- Business reports and invoices
A4 in Millimetres, Inches, and Pixels
At 300 DPI (standard print quality), A4 is 2480 × 3508 pixels. At 72 DPI (screen resolution), it is 595 × 842 pixels. At 150 DPI (draft quality), it is 1240 × 1754 pixels.
A4 vs US Letter
The main alternative to A4 is US Letter, which measures 216 × 279 millimeters (8.5 × 11 inches). Letter is slightly wider and slightly shorter than A4. Printing A4 on Letter paper often requires scaling to 94–97% to avoid clipping. This mismatch is a constant source of minor printing headaches in a globalised world.
Why A4 Matters
A4 is not just a paper size — it is a piece of infrastructure that quietly underpins modern office work, education, and publishing. Its √2 geometry is elegant, its dimensions are standardized across dozens of countries, and its compatibility with printers, folders, and envelopes is unmatched.