PPI vs DPI: what's the difference, and which one is your screen actually using?

How this is calculated

PPI is a physical property of a display: take the diagonal pixel count (sqrt(width² + height²)) and divide by the diagonal size in inches. A 27-inch 1440p monitor has sqrt(2560² + 1440²) ≈ 2938 pixels across 27 inches, so 109 PPI. Bump the resolution to 4K on the same 27-inch panel and the PPI jumps to 163. The math is unambiguous and the number doesn't change unless you physically change the panel. DPI for printers describes how many discrete ink dots a printer places per inch. A 600 DPI laser printer lays down 600 dots per inch, a 1200 DPI photo printer lays down 1200. Higher DPI generally means finer detail, but the relationship isn't 1-to-1 with image resolution because multiple dots can blend to form a single pixel of color. Designers working for print care about DPI; designers and shoppers working for screens care about PPI. The third, unrelated DPI is the one gaming mice report. Mouse DPI (also called CPI, counts per inch) is how many sensor counts the mouse emits per inch of physical movement. A 1600 DPI mouse moved one inch sends 1600 counts to the computer. Higher DPI doesn't mean more accuracy, it means more counts for the same motion, which you then scale in software. Most competitive players run 400-1600 DPI and adjust sensitivity separately. The reason the confusion persists is historical: Windows calls its display scaling 'DPI scaling' even though it's regulating PPI-equivalent logical sizes, and Photoshop still labels its image resolution field 'Resolution: Pixels/Inch' inside a dialog titled 'DPI'. When someone asks 'what's the DPI of this monitor', they almost always mean PPI. The answer below uses PPI throughout for screens and reserves DPI for printers and mice.

Verdict

PPI describes a screen's sharpness (pixels per inch). DPI describes a printer's dot density or a mouse's sensor counts per inch. They share the word 'per inch' and nothing else. When a monitor spec sheet, Windows scaling slider, or Photoshop dialog mentions DPI for a screen, it really means PPI. Use the PPI calculator to get the real pixel density of any display.