Regional Guides
DIN Paper Sizes Explained β A4, A5, Letter, and the Beautiful Math
Outside the US and Canada, the world uses DIN A-series paper. A4 is the office default, A5 is the notebook standard, A0 is the wall poster β and they all share one elegant mathematical property: cut any A-size paper in half and you get the next size down with the exact same proportions. Here's why that matters and which size to use when.
DIN A-Series Sizes (Most Common)
| Size | mm | inches | Typical Use |
|---|---|---|---|
| A0 | 841 Γ 1189 | 33.1 Γ 46.8 | Posters, blueprints |
| A1 | 594 Γ 841 | 23.4 Γ 33.1 | Large posters, drawings |
| A2 | 420 Γ 594 | 16.5 Γ 23.4 | Medium posters, calendars |
| A3 | 297 Γ 420 | 11.7 Γ 16.5 | Tabloid, presentations |
| A4 | 210 Γ 297 | 8.3 Γ 11.7 | Standard documents, letters |
| A5 | 148 Γ 210 | 5.8 Γ 8.3 | Notebooks, brochures |
| A6 | 105 Γ 148 | 4.1 Γ 5.8 | Postcards, flyers |
| A7 | 74 Γ 105 | 2.9 Γ 4.1 | Tickets, business cards |
Why That Strange Ratio? (1 : β2)
Every A-size has the **1 : β2** aspect ratio (about 1 : 1.414). The reason: when you cut it in half, the resulting rectangle keeps the same ratio. So A4 cut in half is two A5s, each with the identical proportions.
This means a document can be scaled between any A sizes with **zero distortion** β no awkward cropping when you photocopy A4 β A3, or print A5 β A4.
DIN A vs US Letter β Side by Side
| Size | Width Γ Height (mm) | Width Γ Height (in) |
|---|---|---|
| A4 | 210 Γ 297 | 8.3 Γ 11.7 |
| US Letter | 216 Γ 279 | 8.5 Γ 11.0 |
| A5 | 148 Γ 210 | 5.8 Γ 8.3 |
| US Half-Letter | 140 Γ 216 | 5.5 Γ 8.5 |
| A3 | 297 Γ 420 | 11.7 Γ 16.5 |
| US Tabloid | 279 Γ 432 | 11.0 Γ 17.0 |
Resize PDFs Without Cropping
Need to convert an A4 PDF to Letter or vice versa? Our PDF tools handle the resize cleanly with no content loss.
β tools.reduce-pdf-size.title
Which Size for What?
**A4 (or US Letter)**: Standard for almost every document β invoices, contracts, school reports, printed forms.
**A5**: Travel journals, planners, half-page brochures. Half the area of A4.
**A3**: Presentations, posters at trade booths, large tables that don't fit A4.
**A6**: Postcards (in most countries), small flyers, recipe cards.
**A7**: European business cards (smaller than the US 85 Γ 55 mm standard).
Frequently Asked Questions
Why doesn't the US use A4?
Historical inertia. US Letter (8.5 Γ 11 in) was standardized in the 1920s, before the ISO/DIN system was widely adopted. Canada uses Letter too. Mexico uses both. Most other countries use A4.
Can my printer handle both A4 and Letter?
Almost certainly yes β modern printers auto-detect or have settings for both. The tricky part is software: Word documents created on A4 will reformat slightly when opened on Letter (and vice versa).
What's B5? Is it bigger than A4?
B5 is 176 Γ 250 mm β between A4 and A5. The DIN B-series sizes fall between A-sizes. Common for books and notebooks in Japan and Europe.