The short version
PPI is pixels per inch: how densely the pixels of an image or a screen are packed. DPI is dots per inch: how many ink dots a printer lays down. The two get used interchangeably, and for most practical purposes the number you are asked for is PPI, even when the box says DPI.
Pixel density of a screen
A display's PPI comes from its resolution and its diagonal:
PPI = √(width² + height²) ÷ diagonal in inches
A 1920 × 1080 panel on a 24 in monitor is √(1920² + 1080²) = 2,202.9 pixels across the diagonal, ÷ 24 = 91.8 PPI. The same resolution on a 15.6 in laptop is 141 PPI, which is why text looks crisper on the laptop. Phones sit in the 400 to 500 PPI range, far beyond what the eye resolves at arm's length.
PPI of an image file
An image file is just a grid of pixels; it has no physical size until you give it one. The PPI is a tag that says how big to print it:
print size = pixels ÷ PPI
A 3000 × 2000 photo at 300 PPI prints 10 × 6.7 in. The same file at 150 PPI prints 20 × 13.3 in, with no change to the file at all. Changing the PPI tag alone never adds detail.
What resolution do I need to print?
- 300 PPI: photo prints and anything held in the hand. The usual target.
- 150 to 200 PPI: posters and signs seen from a metre or more.
- 100 PPI or less: billboards and banners, viewed from far enough that it does not show.
Working backwards, a 4 × 6 in print at 300 PPI needs 1200 × 1800 pixels, which almost any phone beats. An A2 poster at 200 PPI needs about 3,300 × 4,700.
Where DPI really means dots
An inkjet quoting 1200 or 4800 DPI is describing ink droplets, and it needs several of them to mix one pixel's colour. That is why a 1200 DPI printer is still happy with a 300 PPI image. Scanners are the other place the number is literal: scanning at 600 DPI genuinely samples 600 points per inch of the original, and that is the figure to raise when you want to enlarge a small photo or a negative.
Why screen PPI does not change web images
A web image is measured in CSS pixels, not inches, so its PPI tag is ignored. What matters on a high density screen is supplying enough actual pixels, usually two or three times the layout size, which is what the 2x and 3x versions of an asset are for.
Tools
The PPI Calculator gives the density, dot pitch and aspect ratio of a screen from its resolution and size. Resize Image changes the pixel dimensions, Crop Image trims a photo to the shape a print needs, and the Aspect Ratio Calculator keeps the shape right.