When you need this conversion
Millimeters are the working unit of print in most of the world: paper sizes, margins, bleed, and packaging dielines are all specified in mm. Design tools, screenshots and cameras give you pixels. Converting tells you how much physical space your pixels will fill, or whether a mockup made on screen will fit the printed format.
Picking the DPI
- 96 DPI when the pixels are CSS pixels, for example checking how a web layout prints.
- 300 DPI for anything printed and held in the hand. This is what print shops mean when they say “high resolution”.
- 150 DPI for posters and roll-up banners seen from a few meters away.
- 72 DPI for older Mac and PostScript workflows, and some PDF export defaults.
Sanity check
An inch is exactly 25.4 mm, so at 300 DPI every 300 pixels make 25.4 mm. A 3000 px wide photo prints 254 mm wide, just under the 297 mm long side of an A4 sheet. If your answer is wildly different, the DPI setting is almost always the reason.
Common mistakes
Mixing screen and print DPI. A 1080 px wide graphic is 285.75 mm wide at 96 DPI, but only 91.44 mm at 300 DPI. Decide where it will be used before converting.
Rounding the per-pixel value. Using 0.26 mm instead of 0.264583 mm loses almost 5 mm (4.95 mm) across 1080 px. The converter keeps full precision and only rounds for display.
Going the other way? Use the millimeters to pixels converter.