July 6, 2026
What is DPI? 300 DPI for printing, explained simply
A print shop asks for “300 DPI” and suddenly nobody knows if their photo qualifies. Here is the whole concept in plain language.
DPI only matters on paper
DPI (dots per inch) describes how densely pixels are packed when printed. On a screen, DPI is meaningless — a 1000×1000 pixel image is a 1000×1000 pixel image, and the “DPI” number stored in the file changes absolutely nothing about how it looks online. Websites, forms and apps care about pixels, not DPI.
The one formula you need
When you do print, the math is simple:
pixels = inches × DPI
At the standard 300 DPI, common print sizes need:
| Print size | Pixels needed |
|---|---|
| 4×6 in (10×15 cm) | 1200 × 1800 |
| 5×7 in (13×18 cm) | 1500 × 2100 |
| 8×10 in (20×25 cm) | 2400 × 3000 |
| Passport 2×2 in | 600 × 600 |
| ID photo 35×45 mm | 413 × 531 |
Any modern phone photo (4000+ pixels wide) has plenty of resolution for all of these.
“Changing the DPI” — the myth
Editing the DPI number in a file without changing its pixels does nothing to quality — it is just a label suggesting a print size. What actually matters is having enough pixels for the print you want. If a form or print shop demands a specific size “at 300 DPI”, what they really need is the pixel count from the table above.
That is why PhotoReducer’s document tools work in pixels: the 2×2 inch passport tool outputs exactly 600×600 pixels and the 35×45 mm tool outputs 413×531 — which is 300 DPI at those physical sizes, by definition.
Practical checklist
- Printing at home or a shop? Make sure your image has at least the pixels from the table — resize downward if the file is huge, never upward.
- Uploading to a website or form? Ignore DPI completely; match the pixel and KB limits they ask for — the exact-size compressor handles the KB part.