Change the DPI of an image

How to use it#

1. Drop a PNG or JPG onto the box above, or click it to pick one.
2. Set the resolution with the slider or the box, or take one of the presets.
3. Check the printed size, in inches and centimetres, which is what the number actually decides.
4. Download the file, which keeps the same pixels at the new DPI.

DPI does not change the image, only its printed size#

An image is a grid of pixels, and that grid is the same whether the file says 72 DPI or 300. What the number does is tell a printer or a layout program how tightly to pack those pixels on paper: at 300 dots per inch, a 2400 pixel wide image covers eight inches, and at 72 it covers a little over thirty three.

Nothing about the picture changes, which is why setting the DPI takes a few milliseconds and why the file you get back is within a few bytes of the one you put in. On a screen the number is ignored entirely, so a 300 DPI file and a 72 DPI file of the same image are indistinguishable in a browser.

Why 300 keeps being the number you are asked for#

Three hundred dots per inch is roughly where printed detail stops being visible at normal reading distance, so it became the default request from journals, print shops, conference organisers, and anyone laying out a page. Newspapers often accept 200, fine art printing sometimes wants 600, and a poster read from across a room can look fine at 150 because nobody stands close enough for it to matter.

Screens do not use the number at all. A photograph for a website is measured in pixels, and its DPI field is ignored by every browser, so a request for a 300 DPI image for a web page is almost always somebody asking for something larger and reaching for the wrong word.

Setting 300 DPI does not add detail that was not there#

If a print shop asks for an image eight inches wide at 300 DPI, they are asking for 2400 pixels. Taking a 600 pixel image and setting it to 300 DPI produces a file that prints at two inches, which is correct and also not what anyone wanted. The number went up and the picture stayed small.

Work out what you need before changing anything: multiply the printed size in inches by the DPI. Eight by ten inches at 300 needs 2400 by 3000 pixels. If your file is smaller than that, no DPI setting will fix it, and enlarging it in an editor invents detail rather than recovering it. Start from the largest original you have.

Where the number is stored, and why some programs disagree#

A PNG keeps its resolution in a pHYs chunk, as pixels per metre, which is why a value of 300 DPI is written as 11811 and read back out again. A JPEG usually keeps it in the JFIF header at the front of the file, and a photograph from a camera or a phone often carries a second copy in its EXIF data.

When those two disagree, which program you open the file in decides what you see. Both are written here whenever both are present, so the file does not contradict itself. If a program still shows the old value, it has almost certainly cached a thumbnail rather than reread the file.