DPI vs pixels: what a photo size really means

A digital photo is a grid of pixels: 4032 × 3024 from a typical phone, 1080 × 1080 for a square post. That grid is all the detail there is, and nothing you do afterwards adds more.

DPI (dots per inch, often written PPI for images) is a single number stored in the file that says how densely those pixels should be laid out on paper. A photo marked 72 DPI and the same photo marked 300 DPI are identical pixel for pixel; they would just print at different sizes.

On screens and in uploads it doesn’t. A browser, a phone gallery and an upload form show the pixels and ignore the DPI note entirely. If a portal rejects a photo, the reason is almost always its pixel dimensions or its file size in KB, not the DPI field.

On paper it does. Print size is pixels divided by DPI. At the common photo-print density of 300 DPI, a 4 × 6 inch print needs 1200 × 1800 pixels and an A4 page needs 2480 × 3508. Fewer pixels than that and the print is soft; more is simply scaled down.

Some forms still ask for a DPI value, usually alongside a size in millimeters. What they really want is the pixel count those two imply: 35 × 45 mm at 300 DPI is 413 × 531 pixels.

If it asks for pixels (600 × 600, for example), resize to those pixels. Resize Image sets exact dimensions and lets you drag the frame so the face stays centered.

If it asks for a size in mm or inches plus a DPI, switch the unit in Resize Image to mm or inches and set the DPI under Advanced: it works out the pixels for you and writes the DPI into the file.

If it also has a KB limit, compress afterwards with Compress Image, which aims at the limit instead of guessing a quality.

Before uploading, the Photo Size Checker shows a photo’s pixels, KB, DPI and print size side by side, and checks them against common passport and exam rules.

Raising the DPI of a small photo without adding pixels just makes it print smaller. Resampling it up to more pixels makes the file bigger but cannot invent detail that was never captured, so the result looks smooth rather than sharp. If a print looks blurry, the fix is a photo with more pixels to begin with, not a bigger number in the DPI field.