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One formula, three questions
Pixels, print size and DPI are tied together by one relation: pixels = inches × DPI, with 25.4 mm to the inch. Know two of them and the third follows. The calculator above works it out whichever way round you need:
- How many pixels? A print size and a DPI give the pixels a file must have. A4 at 300 DPI needs 2480 × 3508.
- How large will it print? Pixels and a DPI give the physical size. A 4000 × 3000 photo at 300 DPI covers 33.9 × 25.4 cm.
- What DPI do I get? Pixels and a print size give the resolution the print will really have. The same 4000 × 3000 photo printed on A3 comes out at about 242 DPI.
| Size | Millimetres | At 150 DPI | At 300 DPI |
|---|---|---|---|
| A3 | 297 × 420 | 1754 × 2480 | 3508 × 4961 |
| A4 | 210 × 297 | 1240 × 1754 | 2480 × 3508 |
| A5 | 148 × 210 | 874 × 1240 | 1748 × 2480 |
| US Letter | 215.9 × 279.4 | 1275 × 1650 | 2550 × 3300 |
| Photo 10 × 15 cm | 100 × 150 | 591 × 886 | 1181 × 1772 |
| Business card | 85 × 55 | 502 × 325 | 1004 × 650 |
DPI does not decide file size
This is the mix-up behind searches like “300 DPI to KB”. DPI is a number in the file header; changing it does not add or remove a single pixel, so it does not change the file size either. KB comes from the pixel count and the compression. A4 at 300 DPI is 8.7 million pixels — about 25 MB uncompressed, a few megabytes as a JPEG.
So when a form asks for “300 DPI and under 100 KB”, those are two separate requirements. The photo resizer meets both at once.
DPI or PPI?
Strictly, the number in an image file is PPI, pixels per inch; DPI is how many ink dots a printer lays down. Print shops, forms and software almost always say DPI for both, and so does this page. When a print shop asks for “300 DPI”, it means 300 pixels per inch at the printed size.
To write a DPI value into a JPG or PNG, use the DPI converter; to see what a file declares now, check the DPI of an image; for the images inside a PDF, the PDF DPI checker.
How to calculate DPI, pixels or print size
Step 1: Choose the question
“How many pixels?”, “How large will it print?” or “What DPI do I get?” — each asks for the two values it needs.
Step 2: Enter what you know
Pick a paper size or type your own in mm, cm or inches; type the pixels from the file’s properties; choose a DPI.
Step 3: Read the result
It updates as you type. The pixel answer also shows megapixels and the uncompressed size; the size answer shows the largest paper the pixels cover.
Frequently Asked Questions
How many pixels is A4 at 300 DPI?
2480 × 3508 pixels, portrait. At 150 DPI it is 1240 × 1754; at 600 DPI, 4961 × 7016.
How do I calculate DPI from pixels and size?
Divide the pixels by the size in inches: 3000 pixels across 10 inches is 300 DPI. In millimetres, divide the pixels by (mm ÷ 25.4).
What is 300 DPI in pixels per centimetre?
About 118 (300 ÷ 2.54). A 10 cm wide print at 300 DPI therefore needs about 1181 pixels across.
Does a higher DPI give a better print?
Only when the pixels are really there. Raising the DPI tag of an existing file shrinks the size it claims to print at; it adds no detail.
Is 72 DPI bad?
Not by itself. 72 is a default many programs write, and screens ignore it. For print, what matters is whether the pixels cover the paper at the DPI the print shop wants — which is exactly what this calculator answers.
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