Change the DPI of an image

What DPI really changes

DPI (dots per inch) is a number stored in the file that tells a printer how big to print it. It does not change the pixels. A 1,200 x 1,800 pixel photo is the same image at 72 DPI and at 300 DPI; it prints at 16.67 x 25 inches at 72 and at 4 x 6 inches at 300.

So changing the DPI will not make a photo sharper, and it does not change the file size in any way that matters. In our test, setting 300 DPI left every pixel untouched; the file stayed the same size when it already had a JFIF header, and grew by 18 bytes when one had to be added.

Where the DPI is stored, and why checkers disagree

A JPG can hold its DPI in four places: the JFIF header, the Exif data that cameras and phones write, a resolution block Photoshop adds, and XMP data from editing programs. When they disagree, programs show different numbers for the same file, because each reads its own favourite first; Photoshop, for one, reads its own block. This tool reads all four and lists any that disagree. When it saves, it writes the new DPI into every one the file has, adds a JFIF header if there is none, and names the places it wrote.

A PNG keeps its DPI in a pHYs chunk, counted in pixels per metre: 300 DPI is stored as 11,811 per metre. BMP and TIFF files have resolution fields in their headers, which the tool changes in place. WebP, AVIF and GIF have no DPI field most programs read, so the tool saves those as PNG: the picture as your browser decodes it, in sRGB colour, and for an animated GIF only the first frame.

Changing DPI versus changing size

If a form wants a photo at 300 DPI and a set print size, it needs enough pixels as well. A 3.5 x 4.5 cm photo at 300 DPI needs 413 x 531 pixels. Changing only the DPI of a 276 x 354 photo to 300 would make it print at 2.3 x 3 cm instead.

To set the pixels and the DPI together, use resize in cm and KB. Upload notices that name a DPI, such as the MSTC and Punjab & Sind Bank notices that ask for scans at 200 DPI or more, mostly care that the scan captured enough detail. Changing the number afterwards adds none; scanning at 200 DPI does.

Sources, checked 13 October 2026: JPEG File Interchange Format 1.02 (W3C copy, PDF); PNG Specification, Third Edition: the pHYs chunk (W3C); ExifTool EXIF tag reference (XResolution, YResolution, ResolutionUnit); Adobe Photoshop File Formats Specification (image resource 0x03ED, ResolutionInfo); ExifTool XMP tiff tag reference (XResolution, YResolution, ResolutionUnit); Punjab & Sind Bank, guidelines for scanning photograph, signature, thumb impression and declaration (2024, PDF).