Paper Size Converter

This page pairs dimensions with sheet weight: give it a GSM figure and it tells you what a sheet of that size actually weighs, which is the number that decides postage. For area, aspect ratio and the arithmetic behind why the A-series halves so neatly, see the paper size reference.

Standard paper sizes and their dimensions.

ISO A-series (international):

North American sizes:

C-series envelopes, which exist to hold the A-series:

The A-series is one of the most elegant standards ever designed. Each size is exactly half the one before, and because they all share the 1:√2 ratio, cutting a sheet in half keeps the proportions identical. That is why you can scale an A3 document down to A4 with no cropping and no distortion, and the scaling factor is always the same 70.7% going down or 141.4% going up. Those are the numbers printed on every photocopier reduction button in Europe, and now you know where they come from.

A0 was defined as one square metre, or as close as whole millimetres allow: 841 × 1189 mm works out to 999,949 mm², which is 0.999949 m². That definition is what makes paper weight simple. GSM means grams per square metre, so a sheet of A0 at 80 gsm weighs 80 grams, an A1 weighs 40, an A2 weighs 20, and by the time you reach A4 a sheet weighs almost exactly 5 grams. A 500-sheet ream of 80 gsm A4 comes to about 2.5 kg, which is why the box feels the way it does.

North American Letter and Legal come from a different tradition and share none of that. The practical headache is that A4 (210 × 297 mm) and US Letter (215.9 × 279.4 mm) are close but not equal: A4 is 5.9 mm narrower and 17.6 mm taller. Neither contains the other, so there is no scale factor that maps one onto the other without shrinking. To fit A4 content onto Letter you reduce to 94%; going the other way you reduce to 97%. A document laid out for one can shift its margins or clip a footer when printed on the other, which is the usual culprit when a file prints fine at home and comes out wrong at an overseas office.