What DPI means in an image file
DPI stands for dots per inch. In an image file it is a small note stored next to the picture that says how many pixels should be printed in each inch of paper. It does not describe how much detail the picture has; the pixel dimensions do that. DPI only tells a printer, or a program such as Word or InDesign, how large the picture should come out on paper.
The arithmetic is simple: printed size equals pixels divided by DPI. An image 3000 pixels wide set to 300 DPI prints 10 inches wide. The very same image set to 72 DPI is told to print more than 41 inches wide. The pixels are identical in both cases.
When you need to change it
The usual reason is a requirement. A print shop asks for artwork at 300 DPI. A journal or a university wants figures at 300 or 600 DPI. An application portal or a government form states that the photo or the scanned signature must be 300 DPI, and rejects files that declare 72 or 96. Many phone photos and screenshots carry 72 or 96 by default, or no value at all, even when they have plenty of pixels.
In those cases the image is fine and only the note is wrong. Changing the DPI here fixes the note, and the file passes the check.
The pixels are not touched
This tool writes the new resolution into the file and leaves the picture alone. With JPG and PNG, the image is not recompressed: it stays identical, with the same quality and the same pixel dimensions. Other formats, such as WEBP, GIF or BMP, are converted to PNG so that the value can be stored.
You can choose 72, 96, 150, 300 or 600 DPI, or enter a custom value. When the original file declares its resolution, the tool shows it, so you know what you are changing from. Several images can be set to the same value in one go.
What changing DPI cannot do
Setting a small image to 300 DPI does not make it sharper. It adds no detail and no pixels; it only tells the printer to print the same pixels closer together, which makes the print smaller. A picture 600 pixels wide at 300 DPI prints 2 inches wide, whatever the label says.
If you need a certain printed size at a certain DPI, check whether you have enough pixels: multiply the inches by the DPI. An 8 by 10 inch print at 300 DPI needs 2400 by 3000 pixels. When you need a different number of pixels, use “Resize image”, which can also work in centimeters, millimeters or inches. Keep in mind that enlarging a small picture makes it bigger but not more detailed.
Free and private
The change is made in your browser. Your images are not uploaded to a server and nothing is stored. There is no sign-up, no watermark and no limit on the number of files, and it works on a phone as well as on a computer.