Paper sizes explained
Most of the world prints on ISO A-series paper (A4, A3, A5); the US and Canada use Letter, Legal and Tabloid. The A-series has an elegant logic — fold any sheet in half and you get the next size down, same proportions. US sizes don't share that trick. Here's how both work and which to use.
The A-series logic
The ISO A-series starts with A0, defined as exactly one square metre, with sides in the ratio 1 : √2 (about 1.414). That ratio is the magic: halve the sheet across its long side and the two halves keep the same proportion. So A0 folds into two A1s, A1 into two A2s, and so on down. A4 is the everyday size; A5 is half of it; A3 is double.
Why √2 matters
The 1 : √2 ratio is the only proportion that stays constant when you halve a sheet. It means you can scale a layout between A-sizes without changing the proportions, and it's why A-series enlargement/reduction on a copier uses tidy percentages like 71% and 141%.
A-series dimensions
| Size | mm | inches |
|---|---|---|
| A3 | 297 × 420 | 11.7 × 16.5 |
| A4 | 210 × 297 | 8.3 × 11.7 |
| A5 | 148 × 210 | 5.8 × 8.3 |
| A6 | 105 × 148 | 4.1 × 5.8 |
| DL | 99 × 210 | 3.9 × 8.3 |
US sizes and why they differ
US sizes come from traditional imperial dimensions, not a mathematical ratio. Letter is 8.5 × 11 in, Legal 8.5 × 14 in, Tabloid (Ledger) 11 × 17 in. Because their proportions aren't constant, folding a Letter sheet in half doesn't give you a smaller Letter — the shape changes. That's the practical difference designers feel when moving work between regions.
| Size | inches | mm |
|---|---|---|
| Letter | 8.5 × 11 | 216 × 279 |
| Legal | 8.5 × 14 | 216 × 356 |
| Tabloid / Ledger | 11 × 17 | 279 × 432 |
| Half Letter | 5.5 × 8.5 | 140 × 216 |
Choosing a size — and what else to set
A few practical pointers:
- Design for your audience's region: A4 for most of the world, Letter for North America.
- B and C series exist too — C is sized for envelopes (a C5 envelope holds an A5 sheet, or an A4 folded once).
- Whatever the trim size, add bleed before you send to print — see our bleed guide and generate a correct template with the Bleed & Dieline Generator.
- Size is separate from weight — once you've picked dimensions, choose a stock; our GSM vs lb guide and the paper weight converter help.
Frequently asked questions
Why is A4 the standard size?
A4 (210 × 297 mm) is the ISO A-series everyday sheet, used almost everywhere except North America. Its 1 : √2 proportion lets it scale cleanly to other A-sizes, which makes layouts, folding and copier reduction predictable.
Is US Letter the same as A4?
No. A4 is 210 × 297 mm; Letter is 216 × 279 mm (8.5 × 11 in). Letter is slightly wider and shorter. A document laid out for one will not fit the other exactly, so set the right size before designing.
Why doesn't US Letter halve neatly like A4?
Because US sizes are based on fixed imperial dimensions, not the constant 1 : √2 ratio of the A-series. Folding a Letter sheet changes its proportions, whereas folding any A-size gives the next A-size down.
What size envelope fits an A4 letter?
A DL envelope (110 × 220 mm) fits an A4 sheet folded into thirds; a C4 envelope holds A4 flat; a C5 holds A4 folded once or an A5 sheet. The C-series is designed specifically to hold A-series paper.
The elegance of √2
The A-series is one of those quietly brilliant bits of design — fold any sheet and you get the next size down. Once you see the logic you never forget which is which. Reviewed June 14, 2026.