DPI vs pixels: what image resolution really means

Pixels are an image's actual detail: a 3000 × 2000 photo contains 6 million of them. DPI — for images, strictly PPI, pixels per inch — is a number stored in the file that tells a printer how many of those pixels to fit into each inch of paper. Change the DPI without changing the pixels and the picture on screen stays exactly the same.

The only formula you need

• Print size in inches = pixels ÷ DPI
• Pixels needed = print size in inches × DPI
• From millimetres: pixels = millimetres ÷ 25.4 × DPI

300 DPI is the usual target for photo prints viewed up close; posters seen from a distance can use less. Some examples at 300 DPI:

Run the formula backwards to see what an image you already have can do: a 1200 × 1800 photo prints sharply at 4 × 6 inches, or at 8 × 12 inches if 150 DPI is acceptable for the viewing distance.

Why 72 DPI and 300 DPI look the same on screen

Screens don't use the DPI value. A browser or photo viewer draws the image's pixels onto the screen's pixels, scaled to fit the layout; the number in the file is ignored. Two copies of a photo, one tagged 72 DPI and one tagged 300 DPI, are the same image with a different label. The old advice that "web images must be 72 DPI" has never affected how an image looks or how much it weighs.

What does matter on screens is pixel dimensions against display size. High-density phone and laptop screens use two or three physical pixels for each layout pixel, which is why web images are usually prepared at about twice the size they are displayed.

When a form asks for "300 DPI"

Upload forms that ask for a DPI usually mean "enough pixels to print at this size". Check whether the form also gives pixel dimensions — that is the requirement that actually matters. If it only gives a DPI and a print size, work out the pixels with the formula above.

DPI versus PPI

Strictly, PPI (pixels per inch) describes an image and DPI (dots per inch) describes a printer, which uses several ink dots to reproduce each pixel. In everyday use — and in most software, including image file headers — the two are used interchangeably.

Working with pixels and DPI in ImageDoctor

• The Image Resizer changes the pixel dimensions — exact sizes, percentages or a maximum box.
• The Passport Photo Maker and ID Photo Maker work in millimetres or inches, produce the pixels needed at print resolution and write the DPI into the file, so it prints at its real size.
• PDF to Image lets you choose the DPI a page is rendered at, which sets how many pixels the output has.
• The Image Analyzer shows an image's pixel dimensions and megapixels.