What is DPI? A plain guide for forms and printing
DPI and PPI explained: how pixels, inches and DPI relate, why forms ask for 300 DPI, and worked examples for common photo sizes.
DPI in one sentence
DPI (dots per inch) says how many pixels should fit into one inch when an image is printed. A digital image has only pixels; the DPI is a note stored in the file that tells a printer or form how large those pixels should be on paper. PPI (pixels per inch) is the more precise name for the same idea, but forms usually say DPI.
The formula
Pixels = inches × DPI. Since one inch is 25.4 mm, pixels = mm ÷ 25.4 × DPI, rounded to the nearest whole pixel. For centimetres, multiply by 10 first to get millimetres.
Worked examples
A 35 × 45 mm photo at 300 DPI is 35 ÷ 25.4 × 300 = 413 px wide and 45 ÷ 25.4 × 300 = 531 px tall. A 2 × 2 inch photo at 300 DPI is 600 × 600 px. A 3.5 × 4.5 cm photo at 200 DPI is 276 × 354 px.
Why forms ask for DPI
When a form gives a size in cm, mm or inches, it needs a DPI to know how many pixels you mean. Some systems also read the DPI stored in the file and check it. 300 DPI is a common choice for print-quality photos, but always use the DPI your own form states.
DPI does not make a photo sharper
Changing only the DPI number does not add detail. A 600 × 600 px image is the same image whether it is marked 72 or 300 DPI; only its printed size changes. What matters for sharpness is the pixel size and how clear the original photo was.
How PhotoSize handles DPI
Choose mm, cm or inch as the unit, enter the size and the DPI, and the tool calculates the pixels with the formula above. The output has exactly that pixel size, and the DPI is written into the JPEG header so programs that read it see the right value. If you only need pixels, choose px and the DPI field is not used.
Quick converters
To check a number before you start, use /cm-to-pixels/, /mm-to-pixels/ or /inch-to-pixels/. They use the same rounding as the tool, so the pixels match what you get.
Questions
How many pixels is 35 × 45 mm at 300 DPI?
35 × 45 mm at 300 DPI is 413 × 531 pixels. The calculation is mm ÷ 25.4 × 300, rounded to the nearest pixel.
Does changing DPI reduce file size?
No, the DPI value alone doesn’t change the file size. File size depends on the pixel size and the compression; DPI only changes how large the image prints.
Is DPI the same as PPI?
For digital images, yes in practice: both mean how many pixels go into one printed inch. Strictly, DPI refers to printer dots and PPI to image pixels, but forms and most software use DPI for both.
Updated 2026-11-06