280 paper sizes — A4 at 300 dpi is 2480×3508

Every size is the one above, folded

Fold an A3 along its long side and you have an A4; fold that and you have an A5. Folding 420 gives 210; folding 297 gives 148, not 148.5 — the leftover millimetre is dropped. The thirty sizes on this page all come from folding three, so there is a rule to know rather than a table to memorise.

One A0 is one square metre

A0 (841×1189) is what you get when you fix the area at one square metre and then impose the √2 ratio. Sixteen A4 sheets make one A0, and because paper weight is printed in g/m², the weight of a single sheet falls straight out — that is why an 80 g A4 weighs 5 grams. B0 has a short side of exactly one metre, and C0 sits between the two.

The copier’s 141% is this ratio

For the shape to survive folding, the long side must be √2 — 1.414 — times the short one. The 141% that turns A4 into A3, and the 71% that turns A3 into A4, are that number and its inverse. US Letter has no such property, so its enlargement ratios differ from size to size.

Print wants 300 dpi, screens use 96

Resolution is how many dots make up an inch. Divide the millimetres by 25.4 to get inches, multiply by that number, and you have pixels. A print-ready A4 is 2480×3508 because 210 mm is 8.27 inches and 8.27 × 300 is 2480. Double the resolution and the pixel count quadruples.

Why an A4 sheet weighs 5 grams

Paper weight is printed as g/m² — grams for one square metre. An A4 is a sixteenth of an A0, so 0.0625 m²; at 80 g that is 5 grams. If a letter must stay under 25 grams, 80 g paper gives you five sheets.

Letter is shorter and wider than A4

Letter is 8.5×11 inches, or 215.9×279.4 mm. That is 6 mm wider and 18 mm shorter than A4, so a document laid out for A4 loses its bottom or its margins when printed on Letter. Sizes defined in inches do not keep their ratio when folded.

A series — paper

B series — posters and books

C series — envelopes

sizes defined in inches

How to read this

• Dimensions are in millimetres. Leftover fractions are dropped when folding, exactly as the standard says.
• Pixel counts are rounded. Printers usually ask for 300 dpi.
• The weight is the paper alone. Add the envelope and the ink separately.
• Letter and Legal are defined in inches, so folding them does not preserve the √2 ratio.