When the size is given on paper, not on screen
Many instructions describe an image by its printed size: a 4 × 6 inch photo, a logo 5 cm wide for a letterhead, a 2-inch image for a badge, a picture of 10 × 15 cm for an album, an illustration that must fill 180 mm of a page. An image file, though, is measured in pixels, and the same picture can print at any size. The link between the two is the resolution, written as DPI (dots per inch).
This page does the conversion for you. Type the size in inches, centimeters or millimeters, choose the DPI, and the tool calculates the pixels: at 300 DPI, 4 inches become 1200 pixels and 10 cm become about 1181 pixels. Then it resizes the image to those pixels and writes the DPI into the file.
Which DPI should you use?
300 DPI is the usual figure for photos and anything printed to be looked at up close, such as flyers, invitations, books and photo prints, and it is the default here. 150 DPI is often enough for posters seen from a distance or for documents printed at the office. 72 or 96 DPI is a common figure for images that will only be seen on screen. You can type any value from 10 to 2400 if a print shop asks for something specific.
The DPI is saved inside JPG and PNG files, so a print program or a word processor that reads it will place the image at the size you chose without any extra adjustment.
Proportions are kept
The image is never stretched. If you type only the width, the height follows the original proportions, and the other way around. If you type both, the image is fitted inside that box, so one side may come out shorter than the box when the shapes differ. To cut the image to an exact shape first, use “Crop image” and then come back here.
Keep in mind that a small image has limited detail. If your picture has fewer pixels than the print size needs, the tool will enlarge it to reach the size, but enlarging does not add detail: the print will look softer. A photo straight from a phone camera usually has plenty of pixels for common print sizes.
Private and free
The resizing happens in your browser. Your images are not uploaded and nothing is stored, and the results come out without EXIF data, so the location saved by your phone doesn't travel with them. It is free, with no account and no limit, and you can process many images at once and download them in a ZIP.
If you only need to change the DPI label without touching the pixels, use “Change image DPI”. For passport and ID photos at standard sizes, “Passport photo maker” also builds a sheet of copies to print.