How to change an image’s DPI
- Drop a PNG, JPEG or WebP on the box above (up to 50 MB).
- Read the current DPI and the size the image would print at. “Not set” means the file carries no DPI, and most apps will treat it as 72.
- Pick a new DPI — 300 is the common print value — or type your own and press Set.
- Leave the mode on Keep pixels unless you know you want the image resized, then click Download image.
What DPI actually controls
A digital image has only pixels. DPI — dots per inch — is a single number saved alongside them that says how many of those pixels should go into each printed inch. The print size follows directly: width in inches equals width in pixels divided by DPI. A 1200 × 900 pixel image prints at 16.7 × 12.5 inches at 72 DPI, and at 4 × 3 inches at 300 DPI. Same picture, same detail, different paper size.
That is why print shops and publishers ask for “300 DPI”: they want enough pixels per inch that the dots aren’t visible when you hold the page. What really decides whether you can print at 300 DPI is the pixel count. Changing the number on an image that is too small just makes it print smaller.
On screens the number does nothing. Browsers, slide software and chat apps place an image by its pixels, so a web or UI image looks the same at any DPI.
Pixels you need for common print sizes
| Print size | At 300 DPI | At 150 DPI |
|---|---|---|
| 4 × 6 in (10 × 15 cm photo) | 1200 × 1800 px | 600 × 900 px |
| 5 × 7 in | 1500 × 2100 px | 750 × 1050 px |
| 8 × 10 in | 2400 × 3000 px | 1200 × 1500 px |
| US Letter, 8.5 × 11 in | 2550 × 3300 px | 1275 × 1650 px |
| A4, 210 × 297 mm | 2480 × 3508 px | 1240 × 1754 px |
Inches × DPI = pixels. A4 is 8.27 × 11.69 inches. If your image has fewer pixels than the 300 DPI column, it will either print smaller than the target or need enlarging, which softens it.
Keep pixels or keep print size?
Keep pixels is what most people want. It rewrites the DPI and nothing else, so the downloaded file is the same image: no recompression, no quality loss, and every other piece of metadata — camera details, dates, location — left alone. Use it when a form, a print shop or a layout app insists on a particular DPI.
Keep print size resizes the image so it prints at the same physical size at the new DPI. Moving a 1200 × 900 image from 72 to 300 DPI makes it 5000 × 3750 pixels. The new pixels are interpolated from the old ones, so the result is larger but not more detailed, and because it is a fresh file, camera metadata is not carried over. It is useful when software sizes a placed image by its DPI and you want it to arrive at a known width.
Where the DPI is stored — and how we checked
PNG keeps resolution in a small chunk called pHYs, measured in pixels per metre; a PNG without that chunk has no DPI at all. JPEG can store it in two places: the JFIF header, and the EXIF data written by cameras and phones. Many photos carry only EXIF, and some carry both — if the two disagree, one app reads one value and another app the other. This tool updates every copy it finds and adds a JFIF header when a JPEG has none.
We tested it on a PNG with no DPI, a real macOS screenshot, a JPEG at 72 DPI and a Fujifilm camera photo that stores DPI only in EXIF. After setting 300 DPI, both macOS’s own image engine (sips, built on the same ImageIO framework as Preview) and ImageMagick read 300 back from all four, the pixel data was identical to the original, and the files grew by 0 to 21 bytes.
