Image Resolution, DPI and PPI Explained in Plain English

A digital picture is a grid of pixels, such as 4000 by 3000. That pair of numbers is its resolution, and it is the only thing that tells you how much detail the file contains. A screen simply shows those pixels; it does not care about DPI.

PPI means pixels per inch and describes how densely pixels are packed in a given physical size. DPI means dots per inch and strictly refers to ink dots from a printer. People use them interchangeably, and in an image file the stored 'DPI' is only a label saying how large the picture should print by default.

That is why you can change the DPI number of a file without changing a single pixel. The picture looks the same on screen; only the suggested print size changes.

For a sharp print, aim for about 300 pixels per inch. Multiply it out: a 4 by 6 inch print needs 1200 by 1800 pixels, a letter-size page needs 2550 by 3300 pixels, and an A4 page needs 2480 by 3508 pixels. Large posters viewed from a distance can go as low as 100 to 150 pixels per inch.

A full-width banner on a website rarely needs more than 1920 pixels across, and a blog image 1200 pixels. For a profile picture 400 to 800 pixels is plenty. Higher screens can use double that size for crispness, but beyond it you only add weight.

If someone asks for 300 DPI and your picture is only 600 by 400 pixels, changing the label to 300 just says it prints at 2 by 1.3 inches. To print larger you need more real pixels, and software cannot invent detail that was never captured. Enlarging a small image only makes it softer.

Always shrink from a large original rather than enlarging a small copy.