Change an image to 300 dpi without losing quality

The DPI written in a JPEG, PNG or TIFF is a label in the header that says how many of the picture’s pixels go to an inch of paper. It changes nothing on screen and adds no detail. The size a picture prints at is its pixels divided by its DPI, so a 3000 × 2000 pixel photo is 10 × 6.7 inches at 300 dpi, whatever the header says now. If the picture has enough pixels for the print size you need, change the label without resampling and nothing about the picture changes. If it does not, changing the label only makes the print smaller, and the honest answer is a smaller print or a bigger original. DPI rewrites the label in place in the browser tab, without re-saving the picture.

Work out whether you have enough pixels

Multiply the print size in inches by 300. For centimetres, divide by 2.54 first.

Most phone cameras take pictures of 12 megapixels (4032 × 3024) or more, which is 13.4 × 10 inches at 300 dpi. A picture saved from a website or a messaging app is often 1000 to 2000 pixels on its longest side, which is 3.3 to 6.7 inches.

300 dpi is the figure for photographs and fine print viewed at reading distance. Posters and banners viewed from a few feet away print well at 150 dpi or less, which is why a large-format printer may accept a file that "fails" a 300 dpi check.

Changing the label

In Photoshop, Image → Image Size, untick Resample, and type 300 in Resolution: the width and height in inches change and the pixel dimensions stay put. In GIMP, Image → Print Size does the same. Saving afterwards, though, re-compresses a JPEG, which loses a little quality each time for no reason: the picture itself did not change.

DPI avoids that by editing the header and copying everything else byte for byte:

• JPEG: the density in the JFIF header (it adds a JFIF header if the file has none), the EXIF XResolution and YResolution if they are there, and Photoshop’s own resolution record if that is there. Programs disagree about which of these to believe, so it sets all that exist.
• PNG: the pHYs chunk. PNG stores resolution in pixels per metre, as a whole number, so 300 dpi is stored as 11,811 and reads back as 299.9994. Some programs show it as 299 or 300 depending on how they round; that is PNG, not an error.
• TIFF: XResolution, YResolution and ResolutionUnit in the first directory.

It takes several files at once, shows the print size each will come out at, and saves them singly or as a zip. The pictures are read on your own device and not sent anywhere. Checking the current DPI and print size is free; saving the changed files is part of OBSCURA Pro, or US$1 to save from one tool for a day.

When the picture is too small

Resampling up, making new pixels in between the old ones, is the only way to give a small picture "300 dpi at A4", and the extra pixels are interpolated, not detail. For a modest enlargement, up to about one and a half times, a good resampling (Photoshop’s Preserve Details, or bicubic) is often acceptable in print. Beyond that, AI upscalers invent plausible detail, which can work for a landscape and is wrong for a document, a face on an ID photo, or anything that has to be a faithful record. Where you can, find the original: the photo on the phone rather than the copy sent by a messaging app, the logo as a vector file rather than a PNG.

What people usually mean by "300 dpi"

Almost always: "give me enough pixels to print this size sharply". A print shop that rejects a 72 dpi file with plenty of pixels is reading the label; relabel it and it passes with nothing changed. A print shop that rejects a 300 dpi file because it is only 600 pixels wide is reading the pixels, and is right. If the file will be printed at a fixed size, it can help to say both numbers when you send it: "3508 × 2480 pixels, set to 300 dpi, for A4."