What Are PPI and DPI? Pixel Density for Screen and Print

Pixels per inch versus dots per inch

PPI and DPI are often used interchangeably, but they describe two different worlds. PPI belongs to screens: it counts the pixels, the individual points of colored light, packed into each inch of a display. DPI belongs to print: it counts the tiny dots of ink a printer lays down per inch of paper. Both are measures of density, telling you how much detail fits in a fixed physical space, and in both cases a higher number means a finer, crisper result.

The distinction matters because the two units are not directly comparable. A printer often uses several ink dots to reproduce one image pixel, since ink comes in a few fixed colors and must dither to simulate the millions a screen can show directly. That is why print DPI figures like 300 or 1200 sound huge next to screen PPI figures near 100 to 500. When someone hands you a DPI requirement for a print job, do not confuse it with the pixel density of the monitor you are designing on.

How to calculate screen pixel density

To find a screen's PPI you need two things: its resolution in pixels and its physical diagonal in inches. First, use the Pythagorean theorem to turn the pixel width and height into a diagonal in pixels: take the square root of width squared plus height squared. Then divide that pixel diagonal by the screen's diagonal in inches. For a 1920 x 1080 panel on a 15.6-inch laptop, the pixel diagonal is about 2203, and 2203 divided by 15.6 gives roughly 141 PPI.

This formula explains why physical size matters as much as resolution. The same 1920 x 1080 stretched across a 32-inch monitor drops to about 69 PPI, coarse enough to see pixels up close, while squeezed into a 5-inch phone it rises past 440 PPI, far too fine to resolve. So a resolution alone tells you nothing about sharpness; only when you pair it with screen size does the density, and therefore the perceived crispness, become clear.

Retina, print targets, and choosing a density

Retina is not a fixed number but a threshold that depends on viewing distance. Apple coined it for displays dense enough that a person with normal vision, at a typical distance, cannot make out individual pixels. A phone held close needs a higher PPI to qualify than a laptop at arm's length or a TV across the room. That is why a 220 PPI laptop and a 460 PPI phone can both be called Retina: the closer you sit, the more density you need for the pixels to disappear.

For print, the common target is 300 DPI at the final printed size, the point at which most people see continuous tone rather than dots. That drives a concrete rule when preparing images: to print a photo 6 inches wide at 300 DPI, you need 6 times 300, or 1800 pixels across. Undershoot and the print looks soft; overshoot and you waste data. Whether for screen or paper, the practical move is to work backward from the density you need and the physical size, then confirm you have enough pixels to hit it.

Frequently asked questions

Are PPI and DPI the same thing?

No, though people often mix them. PPI counts pixels per inch on a screen; DPI counts ink dots per inch in print. They both measure density per inch but apply to different media, and a printer may use several dots to reproduce one pixel.

How do I calculate the PPI of my monitor?

Compute the pixel diagonal with the square root of width squared plus height squared, then divide by the screen's diagonal in inches. For 1920 x 1080 on a 15.6-inch screen: the pixel diagonal is about 2203, and 2203 divided by 15.6 is about 141 PPI.

What DPI should I use for printing?

300 DPI at the final print size is the common standard for photos and detailed work, since most people no longer see individual dots there. Large posters viewed from a distance can use less, such as 150 DPI, because the eye is farther away.

Is a Retina display a specific PPI value?

No. Retina is a threshold tied to viewing distance, not a single number. The closer a device is normally held, the higher the PPI needed to qualify, so a Retina phone has a much higher PPI than a Retina laptop or TV.