What does changing the DPI actually do?
It changes how big the image prints, and nothing else. DPI (dots per inch, also called PPI) is a number stored next to the pixels that tells apps how many pixels to put in each inch of paper. Print size is pixels divided by DPI.
The same 3000 × 2000 pixel photo at different DPI values:
| DPI stored | Print size | Pixels | Detail |
|---|---|---|---|
| 72 | 41.67 × 27.78 in | 3000 × 2000 | Same |
| 150 | 20 × 13.33 in | 3000 × 2000 | Same |
| 300 | 10 × 6.67 in | 3000 × 2000 | Same |
| 600 | 5 × 3.33 in | 3000 × 2000 | Same |
On a screen the DPI number rarely matters. Most apps and websites show an image by its pixels and ignore the stored DPI.
Can I change DPI without losing quality?
Yes, as long as only the metadata changes, which is what this tool does. The risk comes from editors that resample: in Photoshop’s Image > Image Size dialog, changing Resolution with Resample ticked recalculates every pixel, and saving a JPG again re-compresses it. With Resample unticked, Photoshop only changes the stored number, the same as here.
This tool never decodes the image. For each format it edits the resolution fields and copies the rest of the file as it is:
| Format | Where the DPI is stored | What the tool does |
|---|---|---|
| PNG | pHYs chunk, in pixels per metre |
Rewrites it, or adds one before the image data with a correct checksum. Also updates EXIF and XMP copies |
| JPG | JFIF header, EXIF, Photoshop resolution, XMP | Rewrites each one present; adds a JFIF header when there is none |
| TIFF | XResolution, YResolution, ResolutionUnit tags | Rewrites them in every page, in both byte orders; adds them when missing |
| BMP | Header, in pixels per metre | Rewrites the two header values |
| WebP | No field of its own (WebP container spec) | Updates EXIF or XMP if the file has them; otherwise it can’t store a DPI |
| GIF | No field (GIF89a spec) | Nothing to change |
PNG stores pixels per metre, so 300 dpi is saved as 11,811 pixels per metre, which reads back as 299.9994 dpi. Most apps show 300; a few may show 299.999. Across a 10 inch print the difference is less than a hundredth of a pixel.
We checked the output of every format above with ImageMagick 7.1: it reads the new DPI, and the decoded pixels match the original file’s exactly.
How do I make an image big enough to print at 300 dpi?
You need more pixels, and changing the DPI can’t add them. Multiply the print size in inches by 300. An 8 × 10 inch print needs 2400 × 3000 pixels; the calculator above does the sum for any size and DPI.
If your image is short of that, you have three honest options:
- Print it smaller. Set 300 dpi here and look at the print size the tool shows. That is the largest size it prints at 300 dpi.
- Accept a lower DPI. One university print shop says “for images that are to be printed on an 8.5×11-inch sheet or larger, a DPI of 150-300 is required” and that the needed DPI drops as viewing distance grows (Carleton University Print Shop). Posters seen from across a room don’t need 300.
- Go back to the source. Export the artwork again at a larger canvas size, or use the camera’s full-size original instead of a copy sent through a chat app, which often shrinks photos.
Upscaling in an editor or with an AI upscaler makes more pixels, but they are guessed from the ones you have.
Why do print shops ask for 300 dpi?
They want enough pixels for the print to look sharp up close. Boise State University’s print shop puts it as “all images must be at least 300 dpi at 100% print size” (Boise State Printing). The words that matter are “at 100% print size”: the shop is asking for pixels per inch of paper. A file that says 300 dpi but has too few pixels for the size you ordered will print soft.
If you only need to read the DPI a file has now, and see its print size, Check image DPI explains what each value means.