What Is My Screen PPI?
Your display's true pixel density — with the formula, a reference table of common devices, and why browsers get it wrong.
PPI (pixels per inch) measures how many physical pixels fit into one inch of display. The higher the PPI, the finer the detail your screen can show. Knowing your display's real PPI is the key to making anything rendered on it — rulers, print previews, design mockups — match the physical world.
The PPI Formula
Divide the diagonal pixel count by the diagonal size in inches:
PPI = √(W² + H²) ÷ diagonal inches, where W and H are your native resolution.
Worked example: a 27-inch monitor at 2560 × 1440 has √(2560² + 1440²) = √(6,553,600 + 2,073,600) = √8,627,200 ≈ 2938 diagonal pixels → 2938 ÷ 27 ≈ 108.8 PPI.
Common Device PPI Reference Table
| Device class | Diagonal | Resolution | Approx. PPI |
|---|---|---|---|
| Budget laptop | 15.6" | 1920 × 1080 | 141 |
| Compact laptop | 13.3" | 1920 × 1080 | 166 |
| MacBook Air 13" | 13.6" | 2560 × 1664 | 224 |
| Office monitor | 24" | 1920 × 1080 | 92 |
| QHD monitor | 27" | 2560 × 1440 | 109 |
| 4K monitor | 27" | 3840 × 2160 | 163 |
| Ultrawide monitor | 34" | 3440 × 1440 | 110 |
| iPhone Pro-class | 6.1–6.3" | 2556 × 1179 | ~460 |
| Android flagship | 6.1–6.8" | 2340 × 1080 | ~400–500 |
| iPad Air-class tablet | 10.9" | 2360 × 1640 | 264 |
Why Browsers Assume 96 PPI
CSS defines a reference pixel as exactly 1/96 inch. It is a sizing convention chosen decades ago for consistency across devices — not a measurement of your hardware. On top of that, Windows scaling at 125% or macOS scaled modes decouple CSS pixels from physical pixels even further. The consequence: any web tool that renders 96 CSS pixels per drawn inch is wrong by 30–400% on modern displays.
PPI vs DPI
The terms are used interchangeably but describe different things. PPI counts screen pixels per inch of display; DPI counts printer dots per inch of paper. A 300-DPI printer setting has no effect on your screen's PPI. For measuring on screens, only true PPI matters.
The Easy Way: Skip the Math
You do not need to know your PPI to measure accurately. OpenRulerPro's online ruler, match the credit-card overlay (85.60 × 53.98 mm), and the tool derives your display's effective pixel density automatically — see the fullscreen calibration guide for details.
Frequently Asked Questions
How do I calculate my screen's PPI?
Divide the diagonal pixel count by the diagonal size in inches. For a W × H resolution the diagonal pixels are √(W² + H²). Example: a 27-inch monitor at 2560 × 1440 has √(2560² + 1440²) ≈ 2938 pixels ÷ 27 ≈ 108.8 PPI.
What PPI do most laptops have?
Standard 15.6-inch 1080p laptops run about 141 PPI and compact 13.3-inch models about 166 PPI, while Retina-class laptops exceed 220 PPI.
Why does my browser assume 96 PPI?
CSS defines a reference pixel as exactly 1/96 inch for historical consistency, so browsers report 96 as a fixed convention — not a measurement of your hardware. OS display scaling then further decouples CSS pixels from physical pixels.
Does higher PPI mean better ruler accuracy?
Higher PPI gives finer tick resolution, but measurement accuracy comes from correct calibration, which works identically on any pixel density. Card-based calibration sidesteps PPI math entirely.
What is the difference between PPI and DPI?
PPI (pixels per inch) describes screen pixel density; DPI (dots per inch) describes printer dot density. They are often conflated in marketing, but for on-screen measuring only true PPI matters.
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Last updated: August 2026