Paper sizes fall into two systems: ISO 216 (A-series), used in virtually every country except the US, Canada, and parts of Latin America; and US customary sizes (Letter, Legal, Tabloid), defined in inches. The A-series has a unique property: every size is exactly half the next larger size, with the same 1:√2 aspect ratio, so an A3 sheet folded in half is exactly A4.
The formula
ISO 216 A-series aspect ratio:
width : height = 1 : √2 ≈ 1 : 1.41421
A0 (base) = 841 × 1189 mm (1 m² area, exact)
Each subsequent size: halve the longer dimension
A1 = 594 × 841 mm
A2 = 420 × 594 mm
A3 = 297 × 420 mm
A4 = 210 × 297 mm
A5 = 148 × 210 mm
PostScript points (used in PDF):
1 inch = 72 pt
width_pt = width_mm / 25.4 × 72
height_pt = height_mm / 25.4 × 72
A4: 210 / 25.4 × 72 ≈ 595 pt × 297 / 25.4 × 72 ≈ 842 pt
Practical examples
Example 1 — Printing an A4 document on US Letter. A4 (210×297 mm) is slightly taller (297 vs 279 mm) and narrower (210 vs 216 mm) than Letter. Printing A4 on Letter without scaling clips 18mm from the bottom. Scale to 97% to fit safely.
Example 2 — Setting PDF page size in code. A4 in PostScript points: 595 × 842 pt. In Python with ReportLab: canvas.setPageSize((595, 842)). In PDFKit: { size: 'A4' }.
Example 3 — Choosing paper for a booklet. A5 booklet (148×210 mm) printed two-up on A4: each A4 sheet prints two A5 pages when folded and cut. A booklet of 20 A5 pages requires 5 A4 sheets printed double-sided.
Common mistakes
- Printing A4 documents on Letter without rescaling. Most word processors default to the local paper size. An A4 document emailed internationally often prints with clipped margins. Check the recipient's paper size before sending print-ready files.
- Using Letter dimensions in mm from memory. Letter is 8.5 × 11 inches = 215.9 × 279.4 mm, commonly rounded to 216 × 279. The rounding is acceptable for most purposes but not for precision CNC or die-cut work.
- Confusing Tabloid and Ledger. Tabloid (11 × 17 in) and Ledger (17 × 11 in) are the same sheet — the name reflects orientation: Tabloid is portrait, Ledger is landscape. They are the same paper stock.
International and regional variations
| Name | Width (mm) | Height (mm) | Width (in) | Height (in) | Points (W × H) | Used in |
|---|---|---|---|---|---|---|
| A3 | 297 | 420 | 11.69 | 16.54 | 842 × 1191 | ISO 216 — large prints, engineering drawings |
| A4 | 210 | 297 | 8.27 | 11.69 | 595 × 842 | ISO 216 — global standard office paper |
| A5 | 148 | 210 | 5.83 | 8.27 | 420 × 595 | ISO 216 — notebooks, booklets |
| Letter | 216 | 279 | 8.5 | 11 | 612 × 792 | US, Canada, Mexico |
| Legal | 216 | 356 | 8.5 | 14 | 612 × 1008 | US legal documents |
| Tabloid / Ledger | 279 | 432 | 11 | 17 | 792 × 1224 | US large-format newspapers, spreads |