Paper sizes
Every standard paper size in millimetres and inches, including the R-number photo sizes used in Taiwan and Hong Kong.
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ISO 216 rounds to whole millimetres at every step, so these are the exact cut sizes rather than values derived by halving A0.
| Size | mm | inches |
|---|---|---|
| A0 | 841 × 1189 | 33.11 × 46.81 |
| A1 | 594 × 841 | 23.39 × 33.11 |
| A2 | 420 × 594 | 16.54 × 23.39 |
| A3 | 297 × 420 | 11.69 × 16.54 |
| A4 | 210 × 297 | 8.27 × 11.69 |
| A5 | 148 × 210 | 5.83 × 8.27 |
| A6 | 105 × 148 | 4.13 × 5.83 |
| A7 | 74 × 105 | 2.91 × 4.13 |
| A8 | 52 × 74 | 2.05 × 2.91 |
| B0 | 1000 × 1414 | 39.37 × 55.67 |
| B1 | 707 × 1000 | 27.83 × 39.37 |
| B2 | 500 × 707 | 19.69 × 27.83 |
| B3 | 353 × 500 | 13.90 × 19.69 |
| B4 | 250 × 353 | 9.84 × 13.90 |
| B5 | 176 × 250 | 6.93 × 9.84 |
| B6 | 125 × 176 | 4.92 × 6.93 |
| Letter | 215.9 × 279.4 | 8.50 × 11.00 |
| Legal | 215.9 × 355.6 | 8.50 × 14.00 |
| Tabloid | 279.4 × 431.8 | 11.00 × 17.00 |
| 3R | 89 × 127 | 3.5 × 5 in |
| 4R | 102 × 152 | 4 × 6 in |
| 5R | 127 × 178 | 5 × 7 in |
| 6R | 152 × 203 | 6 × 8 in |
| 8R | 203 × 254 | 8 × 10 in |
| A4 | 210 × 297 | — |
Why A4 folded is A5
Every ISO 216 size has the same aspect ratio: 1 : √2. That is the only ratio with the property that halving the long side gives a rectangle of the same shape, which is what makes the whole series work — A4 folded in half is exactly A5, and enlarging A4 to A3 is a clean 141%.
The series starts from A0, defined as one square metre at that ratio. Everything else is halving.
The sizes are rounded, and that matters
Halving A0 four times gives 210.224 mm for A4. The standard rounds to whole millimetres at every step, so A4 is 210 × 297 exactly, and paper is cut to the rounded value.
Deriving the sizes by repeated division — which some tools do — produces numbers that are close and wrong. These are the standard’s own values.
US sizes have no system
Letter is 8.5 × 11 inches and Legal is 8.5 × 14, so they are not the same shape as each other, and neither is √2. A US document cannot be scaled to A4 without either cropping or leaving a margin, which is why printing an American PDF in Taiwan usually clips the bottom of the page.
The nearest equivalent to A4 is Letter, which is 6 mm wider and 18 mm shorter.
Photo sizes: 3R, 4R, 5R
Print shops in Taiwan, Hong Kong and much of Asia size photographs by R number rather than by inches. 4R is the standard print — 4 × 6 inches, or 102 × 152 mm — and is what a photo lab means by “one print”.
The R sizes come from imperial dimensions but are not named in them, so a shop asking for 5R and a form asking for 5 × 7 inches want the same thing. That mismatch is why the table carries both columns.
Note that none of the R sizes matches the 3:2 aspect ratio most cameras shoot at, so something is always cropped.