What PPI means
PPI stands for pixels per inch. It estimates how densely physical pixels are packed into a display. Higher PPI usually means sharper text and images at the same viewing distance.
Resolution is not enough
A 1920 × 1080 phone and a 1920 × 1080 monitor have the same pixel count but very different pixel density because the physical screen sizes are different. That is why diagonal size matters in a PPI calculation.
Viewing distance matters
Moving a display closer makes each pixel occupy a larger visual angle, so pixels become easier to distinguish at the same PPI. Phones need higher density to preserve fine detail at close viewing distances. Moving a TV farther away reduces the apparent size of its pixels; it does not change the physical PPI.
PPI vs DPR
PPI is about physical pixel density. DPR is about how browser CSS pixels map to physical pixels. Both are useful, but they answer different questions about display sharpness and web layout behavior.
PPI comparison examples
The same pixel resolution can feel different when the physical screen size changes.
| Display example | Resolution | Calculated PPI | What to expect |
|---|---|---|---|
| 24 inch monitor | 1920 × 1080 | 91.8 PPI | Larger individual pixels than either example below. |
| 27 inch monitor | 2560 × 1440 | 108.8 PPI | About 18.5% greater density than the 24-inch example. |
| 6 inch hypothetical phone | 1920 × 1080 | 367.2 PPI | Four times the 24-inch example’s density; not a specification for a named phone. |
Calculation: PPI = √(width² + height²) ÷ diagonal in inches. For 1920 × 1080 at 24 inches, √(1920² + 1080²) ÷ 24 = 91.8 PPI. These geometric examples assume square pixels and use nominal diagonals.
Distance example: one pixel at 100 PPI is 0.01 inches wide. Its angular width is approximately 1.72 arcminutes at 20 inches, but 0.86 arcminutes at 40 inches, using 2 × atan(pixel width ÷ (2 × distance)). Closer viewing makes the same pixel easier to distinguish; it does not make a display sharper. This calculation is not an eyesight or universal “Retina” threshold.
Method: square-pixel geometry and angular-size calculation. For the distinction between physical pixel density and viewing angle, see W3C’s reference pixel and viewing-distance explanation, checked September 5, 2026. The CSS reference pixel is not a way to measure your screen’s PPI.