Change Image DPI

The panel reads before it writes. A file dropped here is inspected first, so the number it already carries and the header that number lives in are on screen before you decide what to replace them with.

1. Choose the JPEG or PNG on your device. Drop the file onto the panel, or press Browse images. JPEG and PNG, up to 20 MB. It is read where it already sits, and only the header is read — the picture itself is never decoded.
2. Read What this file says. The readout appears as soon as the file lands: Recorded resolution and which header it came from, Pixel size, and Prints at the recorded DPI. If nothing was ever recorded it says None recorded, which is the usual answer.
3. Type the number into New DPI. New DPI starts at 300. Type the figure you were asked for, or press one of the Common print resolutions chips — Screens 72, Home printing 150, Print 300, Fine print 600.
4. Check the print size before you commit. The last row of the readout recalculates as you type, so Will print at 300 DPI shows the size on paper the new number implies while the old row still shows the size the file claims now.
5. Press Set DPI. The button reads Writing… while the header is rewritten. Every other byte of the file is copied straight across, so the pixel dimensions cannot move.
6. Read the line under the result, then Download image. It names the value the file carried before, the value it carries now, which resolution fields were written, and the print size that implies. Press Download image to save it.

DPI stands for dots per inch, and inside an image file it is a note in the header saying how large the picture is meant to be when it is printed. It is not a property of the pixels. Nothing about the picture data records it, nothing about the picture data changes when you edit it, and two files with identical pixels can carry different numbers.

That is why the readout on this page prints the arithmetic rather than the number on its own. Pixels divided by DPI gives inches, so the value only means something once you know how many pixels there are — and the same value means a different size on paper for every different picture.

A file often carries no value at all. Screenshots, exports from web tools and plenty of camera output leave the field unset, and software that finds nothing there falls back to 72 and displays that, which is where most of the “my image is only 72 DPI” alarm comes from. The readout says None recorded when that is the case, rather than repeating the guess back to you.

Take a 1600 × 1200 photograph. It is 1600 × 1200 in every row below — the file is the same file, and the only thing changing is the label it carries about paper:

Nothing in that table is a change in quality. The 300 DPI row is not sharper than the 72 DPI row; it is the same photograph asked to occupy a quarter of the width, which is what makes it look better on paper. Print it at 22 inches across and the detail is spread thin whatever the header says.

So this page cannot make a small picture into a large print. If a printer needs more pixels than the file has, no number written into a header will produce them. Need more pixels, not a different label? Resize the image — that changes the picture itself, and enlarging a small one softens it, which is the honest trade.

A JPEG can record its resolution twice: in the JFIF header sitting right at the front of the file, and again in the EXIF block as XResolution and YResolution. A PNG can do the same, in its pHYs chunk and in an eXIf chunk. Different software reads a different one first, and a file whose two records disagree reports one number to your design application and another to the shop’s.

That is the failure this page exists to fix, so it writes every resolution field the file actually has — JFIF and EXIF for a JPEG, pHYs and eXIf for a PNG — and the line under the result names the ones it wrote. The header the format calls its own is created when it is missing, since a file with no resolution record at all is the commonest case; an EXIF block is only ever rewritten in place, never invented, because adding one would move every other field in it.

Everything else is copied straight across. The compressed picture data, every other segment or chunk, and anything a phone stapled onto the end all arrive byte for byte as they were, which is why the pixel dimensions cannot change and why a JPEG does not lose a generation of quality to a header edit. The file grows by a few bytes only when a record had to be added where there was none.

The resolution fields are the only ones touched, so the camera model, the date and any GPS coordinates stay exactly where they were. Want the camera data gone as well? Remove the metadata — that is a separate job on a separate page, and it leaves the pixels alone in the same way.

JPEG and PNG, up to 20 MB per file, one file at a time. WebP is not here because the format defines no resolution field at all — there is no number in a WebP to read or to write, and a tool that claims to set one is decoding the picture and making a new file. HEIC is not supported either; convert an iPhone photo to JPEG first and it arrives with a resolution field of its own.

Values from 1 to 10000 are accepted, but very low ones are not worth setting. Some software — libvips, which a great deal of server-side image handling is built on — treats an implausibly low resolution as no resolution at all and reports its own default of 72 instead, so a file written at 1 DPI can come back reading 72. Every figure a print shop, a portal or a publisher actually asks for — 72, 96, 150, 200, 300, 600 — sits far above the range where that happens.

Changing the DPI does not make the file smaller, because no compression decision is revisited; at most it grows by the few bytes of a header that had to be added. If the file is too heavy for wherever it is going, that is a different job: too large to send? Compress the image, then set the resolution afterwards so the number survives.

All of it happens on your own device. The header is read the moment the file lands, the new one is written when you press Set DPI, and both run on your own machine, so the photo is never transmitted and no copy of it exists anywhere else. This page also asks very little of your hardware compared with the rest of the site — nothing is unpacked into raw pixels, because nothing here needs the picture at all.