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.
Compress PDFWhich 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.
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