What Is DPI? Photo Sizes, Pixels and Printing
Calculate print pixels from millimeters and resolution, and avoid confusing a DPI label with real image detail.
By Nexurivo · Updated
Three measurements describe different things
Pixels describe the digital image grid. Millimeters and inches describe physical size on paper. Resolution links the two. In image preparation, a DPI setting commonly means the intended number of image pixels per printed inch; PPI is the more precise term for that relationship. A printer's physical ink dots are a separate property. Changing a resolution label does not automatically make a photograph sharper. You need sufficient captured detail as well as the right output dimensions.
Calculate the required pixel dimensions
For each side, divide millimeters by 25.4, multiply by the chosen resolution and round to a whole pixel. A 30 × 40 mm photo at 300 DPI becomes 354 × 472 pixels: 30 ÷ 25.4 × 300 is approximately 354.33, and 40 ÷ 25.4 × 300 is approximately 472.44. Rounding is necessary because a raster image cannot contain a fraction of a pixel. Tiny physical-size differences from rounding are distinct from accidentally printing with Fit to page enabled.
Metadata is not new detail
A 1200-pixel-wide image corresponds to four inches at 300 pixels per inch. Changing only its resolution tag to 600 describes a two-inch print using the same pixels. Resampling to create more pixels is a separate operation, and interpolation cannot recover facial details absent from the source. A blurry low-resolution original remains limited even when exported with a high DPI setting. Start with a sharp source and avoid using a screenshot as an intermediate original.
Crop before calculating the final output
A photo intended for a 3:4 rectangle should have an appropriate crop before resizing. Stretching a square image to that shape distorts faces and objects. Nexurivo photo tools use the selected physical dimensions and DPI to generate the output grid. Check the composition, including any required head position or margins, before exporting. A digital upload can instead specify pixels directly; follow that requirement rather than substituting a physical-size preset that happens to sound similar.
Verify the paper result
Photo JPEG and PNG downloads include print-resolution metadata, but print applications may ignore it. Select the intended paper size, use Actual Size or 100%, and disable Fit to page. Print a trial and measure it with a ruler. Printer margins can clip a sheet even when its dimensions are correct. The tool's size calculation does not certify passport or application compliance: lighting, background, expression and the authority's current instructions still need a separate review.