How do I change an image to 300 DPI?
Set the DPI field in the file and save. It is a label written into the file header, not a change to the picture, so nothing is re-encoded and no quality is lost. But it only satisfies a printer if the pixel count is already enough: print size in inches equals pixels divided by DPI.
- Work out how many pixels you actually need. Multiply the printed size in inches by 300. A 4×6 inch print at 300 DPI needs 1200×1800 pixels; an A4 page needs about 2480×3508. Check your image against that number before doing anything else.
- Set the DPI value. Drop the file into the DPI tool and pick 300. The change is written to the file header and the image data is copied through untouched — a JPEG does not lose a generation to it.
- If the pixels fall short, resize instead. A 600×400 photo tagged 300 DPI is still a 2×1.3 inch print. Tagging cannot invent detail. Enlarge it with the upscaler first, then set the DPI on the larger file.
- Confirm before sending. Reopen the saved file and check the DPI field reads 300. Printers and submission portals read that header value, which is exactly what the person asking for "300 DPI" is checking.
A file that reports 300 DPI, byte-for-byte identical in image content to the one you started with. The operation is instant regardless of file size, because the pixels are never touched.
DPI is a label, not image quality
This is the misconception behind most of the frustration. An image has a fixed number of pixels. The DPI field says how densely those pixels should be laid down on paper — nothing more. Changing 72 to 300 does not add detail, sharpen anything, or make a file bigger; it changes the size the picture will print at. The same 3000×2400 image is a 10×8 inch print at 300 DPI and a 41×33 inch poster at 72 DPI. Identical pixels, different instruction.
Where the number is actually stored
PNG keeps it in a pHYs chunk, and in pixels per metre rather than per inch — 300 DPI is written as 11811, because an inch is 0.0254 metres. JPEG keeps it in the JFIF APP0 segment as an X and Y density pair. Both live in the header, so setting them is a small edit to a few bytes rather than a re-encode. If a JPEG carries EXIF but no JFIF block at all, a standard APP0 segment is inserted right after the start marker so the density has somewhere to live.
Why the image looks the same on screen
Because browsers, phones and photo viewers ignore the field entirely. A screen has its own pixel density and maps image pixels to it directly. The DPI tag matters at exactly two moments: when something is printed, and when a piece of software checks the number to decide whether to accept your file. Between those, it is inert.
Tools for this job
Frequently asked questions
Does changing DPI reduce image quality?
No. The pixels are copied through unchanged and only the header is rewritten, so there is no re-encoding and no generation loss — not even on a JPEG, which normally degrades every time it is saved.
Someone asked for 300 DPI. Is setting the tag enough?
Only if the image already has the pixels for the size they intend to print. Ask what physical size it will be printed at, multiply by 300, and compare. If it comes up short, the tag will pass an automated check but the print will look soft.
What about 72 DPI for the web?
It is a leftover convention from early Macintosh displays and has no effect on how an image appears on any modern screen. If a site asks for it, set it to satisfy the form; nothing about the picture changes.
Which DPI should I use for scanning?
For photographic prints, 300 to 600 DPI captures everything the print holds. For line art, text and engravings, 600 is the sensible floor because sharp edges need the extra sampling.