What the PPI calculator does
Enter a screen's resolution in pixels and its diagonal size in inches, and the calculator returns its pixel density in pixels per inch, plus the physical width and height and the dot pitch. PPI is what decides whether text looks crisp or you can see individual pixels, whether a monitor needs display scaling, and how a 27-inch 4K compares with a 32-inch 4K. Runs in your browser.
The formula
Diagonal in pixels = √(width² + height²)
PPI = diagonal pixels ÷ diagonal inches
Dot pitch (mm) = 25.4 ÷ PPI
2560 × 1440 on 27 in: √(2560² + 1440²) = 2937 px; 2937 ÷ 27 = 108.8 PPI; pitch 0.233 mm
3840 × 2160 on 27 in: 4406 ÷ 27 = 163 PPI
2556 × 1179 on 6.1 in (iPhone): 2815 ÷ 6.1 = 461 PPICommon displays
| Display | Resolution | Size | PPI | Notes |
|---|---|---|---|---|
| Full HD monitor | 1920 × 1080 | 24 in | 92 | Standard density; no scaling |
| Full HD monitor | 1920 × 1080 | 27 in | 82 | Visibly soft text |
| QHD monitor | 2560 × 1440 | 27 in | 109 | The productivity sweet spot at 100% |
| 4K monitor | 3840 × 2160 | 27 in | 163 | Needs 150% scaling; very sharp |
| 4K monitor | 3840 × 2160 | 32 in | 138 | 125–150% scaling |
| 5K monitor | 5120 × 2880 | 27 in | 218 | Exactly 2× of QHD — Retina-class |
| MacBook Air 13 | 2560 × 1664 | 13.6 in | 224 | 2× scaling |
| Windows laptop | 1920 × 1080 | 14 in | 157 | 125% scaling |
| iPad Pro 11 | 2420 × 1668 | 11 in | 264 | |
| Flagship phone | ≈ 2400 × 1080 | 6.5 in | ≈ 405 | |
| 4K TV | 3840 × 2160 | 55 in | 80 | Fine — viewed from metres away |
What PPI means in practice
- Under 100 PPI on a desktop monitor at arm's length: pixels are visible, text is soft, 100% scaling.
- 100–120 PPI: comfortable at 100% scaling with sharp text; the most common choice for work.
- 140–170 PPI: too small at 100%; run 125–150% scaling and get sharper rendering than a native lower-resolution panel.
- 200+ PPI (“Retina”): individual pixels are invisible at normal distance; 200% scaling gives the layout of a lower resolution with double the sharpness.
- Phones at 400–500 PPI exceed what the eye resolves at 30 cm; beyond ~300 PPI at phone distance the gain is marginal.
Viewing distance changes everything
The eye resolves about 1 arc-minute. At 60 cm (desk), that is roughly 150 PPI to be pixel-invisible; at 30 cm (phone) about 300; at 3 m (TV) only 30. The same 80 PPI that looks coarse on a 27-inch monitor is perfectly sharp on a 55-inch TV across the room. When comparing displays, compare PPI at the distance you will actually use them — or use pixels per degree, which the distance converts to.
PPI, DPI and scaling
- PPI is a physical property of a screen. DPI properly refers to printer dots, but Windows uses “DPI scaling” to mean UI scaling.
- Scaling (125%, 150%, 200%) tells the OS how many physical pixels to use per logical pixel. Integer scaling (200%) is crisp; fractional scaling can blur some apps on Windows and Linux.
- CSS pixels on the web are logical: a 200-PPI display at 2× reports a 1,280-px-wide viewport for a 2,560-px panel. Images need 2× versions to look sharp — the reason for
srcset. - Matching PPI across two monitors keeps windows the same physical size when dragged between them.