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The A, B and C paper series
Most of the world prints on the ISO paper series, and the logic behind it is elegant. The A series starts with A0, a sheet of one square metre, and every smaller size is exactly half the one before it. Fold A0 in half and you get A1. Fold A1 and you get A2. Keep going and you reach A4, the everyday office sheet at 210 by 297 millimetres, and then A5, the common booklet page. Because each size is half the previous one, two A5 sheets make one A4 sheet with no waste, and scaling between sizes is a simple doubling or halving.
The B series sits between the A sizes. B5 is slightly larger than A5, which makes it popular for books that need a little more margin room. The C series is made for envelopes: a C4 envelope holds an unfolded A4 sheet, and a C5 envelope holds an A4 sheet folded once. This converter gives the dimensions of any A, B or C size from A0 to A10, in millimetres and inches, and in pixels at the resolution you choose.
How to use the converter
- Choose the series: A, B or C.
- Choose the size number from 0 to 10.
- Enter the resolution in DPI for the pixel size, or leave it at 300 for print.
- Read the size in millimetres, inches and pixels.
A worked example
Choose A4 at 300 DPI. The converter shows 210 by 297 millimetres, which is 8.27 by 11.69 inches. Multiply the inches by 300 and you get 2480 by 3508 pixels, the pixel size a print-ready A4 image needs at full quality. If you only have a 1240 by 1754 pixel image, that is A4 at 150 DPI, which looks acceptable for a poster viewed at arm’s length but soft for a photo book page viewed up close. The print quality checker on this site turns that judgement into a verdict based on viewing distance.
Now choose C5. The converter shows 162 by 229 millimetres, the envelope size that holds an A5 booklet or a once-folded A4 letter. Choosing the wrong envelope series is a common mailing mistake: an A4 sheet fits a C4 envelope flat, but it does not fit a C5 envelope without folding. When the envelope size and the paper size come from the same series system, the fit is guaranteed by design.
Paper sizes at a glance
- A0: 841 by 1189 millimetres, poster and technical drawing size.
- A1: 594 by 841 millimetres, flip charts and large posters.
- A2: 420 by 594 millimetres, medium posters and art prints.
- A3: 297 by 420 millimetres, small posters and folded A4 spreads.
- A4: 210 by 297 millimetres, letters, reports and everyday printing.
- A5: 148 by 210 millimetres, booklets, flyers and notebooks.
- A6: 105 by 148 millimetres, postcards and pocket notebooks.
Millimetres, inches and pixels together
Designers juggle three units for the same sheet: millimetres for the print shop, inches for US clients, and pixels for the screen. The conversions are fixed. One inch is 25.4 millimetres, so dividing millimetres by 25.4 gives inches. Pixels come from inches times DPI, so the pixel size depends on the resolution you plan to print at. A sheet has no single pixel size: A4 is 2480 by 3508 pixels at 300 DPI but only 827 by 1169 pixels at 100 DPI. Always state the DPI with a pixel size, or the number means nothing. With this converter you can move between all three units in one step, and you can check a file’s pixel dimensions against the print size before it goes to press.
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