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Arithmetic
Print size and DPI calculator
Pixels have no physical size until you decide how many go in an inch. This works the conversion in both directions and tells you whether the result will actually print well.
Edit any field. Pixels drive the printed size, and printed size drives the pixels needed.
Result
DPI is not stored in the pixels
A 1200 by 1800 image is 1200 by 1800 pixels and nothing else. At 300 DPI it prints about 10 by 15 centimetres; at 72 DPI the same file prints about 42 by 63 centimetres and looks terrible. Nothing about the file changed, only the instruction about how densely to lay the dots down.
That instruction can be written into a JPEG or PNG as a density tag, which is what change image DPI does, and print software usually respects it. It never adds detail: retagging a small image as 300 DPI just tells the printer to make it small.
Two useful rules. For photographs, 300 DPI is the standard and 240 is usually indistinguishable. For anything viewed from further away, a poster or a banner, 150 is plenty, because the eye cannot resolve more at that distance.
Common questions
What size can I print from a 12 megapixel phone photo?
Roughly 4000 by 3000 pixels, so about 34 by 25 centimetres at 300 DPI. Comfortably A4, and A3 at a slight stretch.
Should I upscale to reach the pixels I need?
Only as a last resort. Upscaling invents pixels rather than recovering detail, and at more than about 150% it starts to look soft. Printing slightly smaller is almost always the better answer.
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