International paper sizes are built around a useful geometric idea: when a sheet is cut in half parallel to its shorter side, each half keeps the original proportions. The ISO A series turns that relationship into a predictable family, from large display sheets to small cards.
The ISO A series
ISO 216 specifies the trimmed A and B series for writing paper and certain printed matter. In the A series, A0 has an area of one square metre in principle. Each next size is half the preceding sheet: A1 is half of A0, A2 is half of A1, and so on. Dimensions are rounded to whole millimetres as specified by the standard.
The sides have a ratio of 1 to the square root of 2, approximately 1:1.414. That ratio is what survives halving. If an A4 page is cut across its long dimension, the two A5 sheets have the same shape as the original. Scaling an A4 document to A3 requires about 141%; reducing it to A5 requires about 71%. Printers may impose unprintable margins, so proportional scaling does not guarantee edge-to-edge output.
Common dimensions
| Size | Millimetres | Approximate inches | Typical use |
|---|---|---|---|
| A0 | 841 × 1189 mm | 33.11 × 46.81 in | Posters, technical drawings |
| A1 | 594 × 841 mm | 23.39 × 33.11 in | Posters, presentation sheets |
| A2 | 420 × 594 mm | 16.54 × 23.39 in | Small posters, diagrams |
| A3 | 297 × 420 mm | 11.69 × 16.54 in | Charts, folded spreads |
| A4 | 210 × 297 mm | 8.27 × 11.69 in | Letters, reports, office printing |
| A5 | 148 × 210 mm | 5.83 × 8.27 in | Notebooks, leaflets |
| A6 | 105 × 148 mm | 4.13 × 5.83 in | Postcards, small handouts |
Dimensions are conventionally written width × height with the sheet in portrait orientation. Rotating A4 does not create another size; it is still A4, now 297 mm wide and 210 mm high. Inch values above are conversions rounded to two decimal places, not independent definitions. For production, specify the millimetre dimensions.
Using the proportional relationship
The shared aspect ratio is valuable in document systems. A diagram designed for A3 can be reduced to A4 without cropping. Two portrait A4 pages can be arranged side by side on a landscape A3 sheet. A folded A3 sheet produces four A4 pages when printed on both sides and folded once, subject to imposition, binding, bleed, and trimming requirements.
The naming also helps estimate area and mass. A0 is nominally one square metre; A1 is one-half square metre; A2 is one-quarter; A4 is one-sixteenth. With 80 g/m² paper, an A0 sheet is nominally 80 grams and A4 is about 5 grams. Real stock and trimming introduce variation, so this is an estimate rather than a scale-calibration method.
Printing note: a document page size, the physical sheet loaded in a printer, the printable area, and the final trimmed piece are separate specifications. Commercial work may also require bleed beyond the trim edge. Physical cutting and printing tolerances vary.
Letter and Legal are different systems
North American office paper commonly uses sizes defined in inches. US Letter is shorter and wider than A4; neither fits inside the other without some scaling or cropping. US Legal is the same width as Letter but substantially longer.
| Format | Inches | Millimetres | Aspect ratio |
|---|---|---|---|
| US Letter | 8.5 × 11 in | 215.9 × 279.4 mm | 1:1.294 |
| US Legal | 8.5 × 14 in | 215.9 × 355.6 mm | 1:1.647 |
| A4 | 8.27 × 11.69 in | 210 × 297 mm | 1:1.414 |
A Letter document printed at 100% on A4 may lose content at the sides or leave different margins; an A4 document on Letter may lose content at the top and bottom. “Fit to page” usually solves clipping by scaling, but it also changes type and image sizes. Templates that must work in both environments need generous margins and a test on both formats.
Beyond A sizes
ISO 216 also defines the B series, whose sizes sit between adjacent A sizes and can serve posters, books, and other applications. The ISO C envelope series is defined separately in ISO 269. A C4 envelope accepts an unfolded A4 sheet; C5 accepts A4 folded once to A5; and DL, a common business envelope format, accepts A4 folded into thirds. Not every product described as an envelope or photographic size belongs to these series.
Book formats deserve particular care. A finished book may be described informally as “A5,” but publishers often use custom trim sizes, and ISO 216 does not necessarily govern published books. Ask for the actual trim dimensions rather than relying only on a familiar label.
Choosing and specifying a size
For general documents in an ISO-paper environment, A4 is the ordinary starting point. A3 provides twice its area for plans or shared viewing; A5 makes a portable booklet or handout. In a Letter-paper environment, using Letter avoids unnecessary printer scaling. For international distribution, offer a carefully tested PDF for each target size or design a flexible layout with safe margins.
A production instruction should include the series name and dimensions—“A4, 210 × 297 mm”—plus orientation, stock, printing sides, bleed, and final trim where relevant. That small amount of redundancy prevents a named format from being mistaken for an approximate size.