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A4, Letter and other paper sizes in pixels
A design for an A4 flyer, a poster or a printable worksheet starts with a canvas, and the canvas is measured in pixels. Paper is measured in millimetres or inches. The link between them is the resolution, in dots per inch, and getting it right is the difference between a crisp print and a blurry one.
The formula
pixels = inches × DPI
pixels = millimetres ÷ 25.4 × DPI
A4 is 210 × 297 mm. At 300 DPI:
210 ÷ 25.4 × 300 = 2480 px
297 ÷ 25.4 × 300 = 3508 px
So a print-ready A4 page is 2480 × 3508 pixels.
A4 and Letter at common resolutions
| Paper | 72 DPI | 96 DPI | 150 DPI | 300 DPI |
|---|---|---|---|---|
| A5 (148 × 210 mm) | 420 × 595 | 559 × 794 | 874 × 1240 | 1748 × 2480 |
| A4 (210 × 297 mm) | 595 × 842 | 794 × 1123 | 1240 × 1754 | 2480 × 3508 |
| A3 (297 × 420 mm) | 842 × 1191 | 1123 × 1587 | 1754 × 2480 | 3508 × 4961 |
| US Letter (8.5 × 11 in) | 612 × 792 | 816 × 1056 | 1275 × 1650 | 2550 × 3300 |
| US Legal (8.5 × 14 in) | 612 × 1008 | 816 × 1344 | 1275 × 2100 | 2550 × 4200 |
Each A size is half the one above it, folded across the long side, so A5 at 300 DPI is exactly half an A4 page.
Which DPI to use
300 DPI for print. This is the standard for anything people will hold and read close up: flyers, worksheets, photos, books. At normal reading distance, the eye cannot pick out individual dots at 300 DPI.
150 DPI for large prints seen from a distance. Posters and banners are read from a metre or more away, so lower resolution is fine and keeps file sizes manageable. Large-format printers often recommend 100–150 DPI.
72 or 96 DPI for screens only. These numbers come from old conventions for screen layout. They describe nothing physical on a modern display, but tools use them to convert page sizes into on-screen sizes. A 96 DPI A4 canvas is the size a browser uses for an A4 page in print preview.
If in doubt, design at 300 DPI. It is easy to shrink a large image for screen and impossible to add detail to a small one.
The DPI setting versus the pixels
Image files store a DPI value in their header, alongside the pixels. Changing that value does not change the image at all. A 2480 × 3508 photo is the same whether the header says 72 or 300; only the suggested print size changes:
2480 px ÷ 300 DPI = 8.27 in (A4 width)
2480 px ÷ 72 DPI = 34.4 in
This matters because some print shops and submission systems check the header and reject files marked 72 DPI, even when there are plenty of pixels. In that case, rewriting the header to 300 is all that is needed; there is no reason to resample the image.
The reverse is the real problem. A 800 × 600 web image cannot become a good A4 print by changing a setting. At A4 width it works out to under 100 DPI, and it will look soft.
Checking if an image will print well
Divide the pixels by the inches of the print:
effective DPI = pixel width ÷ print width in inches
A 3000 px wide photo printed 10 inches wide gives 300 DPI, which is excellent. Printed 20 inches wide it gives 150 DPI, fine for a poster on a wall. Printed 40 inches wide it gives 75 DPI, which will look soft up close.
Bleed and margins
For professional printing, add bleed: an extra strip, usually 3 mm on each side, that extends the background past the trim line so no white edge shows after cutting. An A4 page with 3 mm bleed is 216 × 303 mm, or 2551 × 3579 pixels at 300 DPI. Keep text at least 5 mm inside the trim line too, since cutting is never perfectly exact.
Work it out
The paper size calculator lists the A, B and North American sizes in millimetres, centimetres, inches and pixels at any DPI you enter. To go from a pixel size to physical size, use pixels to inches, which shows results at 72, 96, 150, 300 and 600 PPI. If a print shop rejects a file for its DPI value, the image DPI changer rewrites the header without recompressing the image, and the print size calculator shows how large a photo will print well.