Pixels per Inch Calculator
Measure how sharp a screen really is. Enter the resolution in pixels and the diagonal size in inches to get pixels per inch (PPI = diagonal pixels ÷ diagonal inches), the dot pitch in millimeters, and the total pixel count.
Example: with Width (px) 1920 · Height (px) 1080 · Diagonal (inches) 24 → Pixel density: 91.79 PPI.
- Dot pitch0.2767 mm between pixel centers
- Total pixels2.07 MP (2,073,600 pixels)
- Density classStandard desktop density — fine at desk distance
Computed by the calculator below using its default values. Change any input to see your own numbers.
PPI = √(width² + height²) ÷ diagonal inches. A 24-inch 1080p monitor is 91.8 PPI; a 27-inch 4K is 163.2 — same panel size class, 1.8× the sharpness.
What PPI actually measures
Pixels per inch is pure geometry: the count of pixels along the screen's diagonal divided by the diagonal's length. The Pythagorean theorem gives the diagonal pixel count — √(1920² + 1080²) ≈ 2,203 for Full HD — and dividing by a 24-inch diagonal yields 91.8 PPI. Its reciprocal, the dot pitch (25.4 ÷ PPI, in mm), is how panel makers state the same fact.
Sharpness in practice is PPI times viewing distance. A 92 PPI desktop at 30 inches subtends about the same pixel angle as a 460 PPI phone at 6 inches — both sit near the limit of what a typical eye resolves. That is why TVs get away with low PPI and phones cannot.
How it’s calculated
PPI = √(width² + height²) ÷ diagonal size in inches, assuming square pixels. Dot pitch (mm) = 25.4 ÷ PPI. Megapixels = width × height ÷ 1,000,000. Density classes are descriptive bands, not standards: under 110 desktop-typical, 110-200 high, 200-300 Retina-class, 300+ phone/print-grade.
Assumes square pixels and that the quoted diagonal is the true visible panel — marketing sizes sometimes round up half an inch, which inflates real PPI slightly.
PPI of common displays
| Display | Resolution | PPI |
|---|---|---|
| 24 in monitor | 1920 × 1080 | 91.8 |
| 27 in monitor | 2560 × 1440 | 108.8 |
| 32 in monitor | 3840 × 2160 | 137.7 |
| 27 in monitor | 3840 × 2160 | 163.2 |
| 13.6 in MacBook Air | 2560 × 1664 | 224.5 |
| 6.1 in phone | 2532 × 1170 | 457.3 |
Computed with √(w² + h²) ÷ diagonal; rounded to 0.1 PPI.
Common mistakes
- Dividing horizontal pixels by the diagonal — you must divide diagonal pixels by diagonal inches, or the result is meaningless.
- Comparing PPI without viewing distance: 92 PPI at arm's length looks as sharp as 300+ PPI held near your face.
- Using the marketing size class (a "27-inch" that is really 26.5) — half an inch shifts PPI by about 2%.
- Calling it DPI: printers lay ink dots (DPI); screens have pixels (PPI). The numbers are not interchangeable.
Frequently asked questions
What is the PPI formula?
PPI = √(width² + height²) ÷ diagonal inches. For a 24-inch 1920 × 1080 monitor: √(3,686,400 + 1,166,400) = 2,202.9 diagonal pixels, ÷ 24 = 91.8 PPI.
What is the difference between PPI and DPI?
PPI counts a display's physical pixels per inch; DPI counts a printer's ink dots per inch. Software often says DPI when it means PPI, but for screens the correct unit is PPI.
What PPI counts as Retina?
Retina is Apple's marketing term for density high enough that pixels vanish at the intended distance — roughly 220 PPI for laptops and 300+ for phones. It is about pixel angle at your eye, not one magic number.
Is higher PPI always sharper?
Only at the same viewing distance. Past the eye's resolving limit (about one arcminute per pixel) extra density is invisible — which is why a 460 PPI phone and a 100 PPI TV can both look perfectly sharp in normal use.