Printers, forms and paper sizes talk in centimeters and inches. Images only know pixels. DPI (dots per inch) is the bridge between them: it says how many pixels fit into one inch of print. The formula is simple, pixels = cm ÷ 2.54 × DPI, so 10 cm at 300 DPI needs 10 ÷ 2.54 × 300 ≈ 1181 pixels.
This page has two parts: a calculator for those conversions and a tool that changes the DPI value saved in your files. Both run in your browser, and files are not uploaded.
How to use it
Calculator
- Type a value in any unit: cm, mm, inches or pixels.
- Choose a DPI: 72, 96, 150, 300, 600 or a custom value.
- Read the other units, which update as you type. Below them you will see common paper sizes (A4, A5 and 10 × 15 cm) in pixels at the chosen DPI.
Change a file's DPI
- Drop one or more JPG or PNG files.
- Choose the DPI you need.
- Check the resulting print size the tool shows for each file, then download.
What changing DPI does and does not do
The DPI number in a file is only an instruction for printing. The tool rewrites the resolution field (the JFIF density in a JPG, the pHYs chunk in a PNG) and nothing else. Pixels, colors and compression stay identical, so there is no quality loss.
That also means a new DPI does not make the image sharper. Sharpness depends on how many pixels the image has. Take a photo of 2400 × 3000 pixels:
| Stored DPI | Print size |
|---|---|
| 300 | 20.3 × 25.4 cm (8 × 10 in) |
| 150 | 40.6 × 50.8 cm (16 × 20 in) |
| 72 | 84.7 × 105.8 cm (33.3 × 41.7 in) |
The picture is exactly the same in all three rows. Only the size it will print at changes. At 72 DPI the print is huge but each pixel is large enough to see; at 300 DPI it is smaller and looks crisp up close.
Many forms say a photo must be 300 DPI. Changing the number satisfies a check that only reads the field, but if the image has too few pixels for the requested print size, it will still print soft. In that case you need a larger original, because enlarging with Resize image cannot add detail that was never captured.
Pixel sizes for common papers
| Paper | 150 DPI | 300 DPI |
|---|---|---|
| A4 (210 × 297 mm) | 1240 × 1754 px | 2480 × 3508 px |
| A5 (148 × 210 mm) | 874 × 1240 px | 1748 × 2480 px |
| 10 × 15 cm photo | 591 × 886 px | 1181 × 1772 px |
300 DPI is the usual target for photos and documents viewed at reading distance. 150 DPI is often acceptable for posters seen from a few steps away. 72 and 96 DPI are screen conventions; screens ignore the stored DPI and show the image pixel for pixel.
To see the DPI and print size of files you already have, use the image size checker. For a full explanation with more examples, read What is DPI.
Frequently asked questions
Does changing the DPI make my photo sharper?
No. It only changes the size the photo prints at. Sharpness comes from the number of pixels, which this tool does not change.
Why does my photo say 72 DPI?
Many cameras, phones and programs write 72 DPI by default, or no value at all. It says nothing about quality. A 4000 × 3000 photo at 72 DPI has exactly the same detail as the same photo at 300 DPI.
Which file types can I change the DPI of?
JPG and PNG. For other formats, convert them first, for example with HEIC to JPG, then set the DPI.
What DPI should I use for printing?
300 DPI for photos and documents you hold in your hand. Lower values such as 150 DPI are often fine for large prints viewed from a distance. If a print shop gives you a value, use theirs.