How PPI (pixel density) is calculated
Dividing the diagonal pixel count by the diagonal size in inches gives pixels per inch (PPI). The diagonal pixel count comes from the Pythagorean theorem applied to the horizontal and vertical resolution.
- widthPx — horizontal resolution (pixels)
- heightPx — vertical resolution (pixels)
- diagonal(in) — screen diagonal size (inches)
Common values
| Diagonal (inches) (in) | Pixel density (PPI) |
|---|---|
| 6 in | 367.2 |
| 12 in | 183.6 |
| 24 in | 91.8 |
| 36 in | 61.2 |
| 48 in | 45.9 |
| 72 in | 30.6 |
| 120 in | 18.4 |
More detail
What a higher PPI means
At the same screen size, a higher resolution means a higher PPI and smaller individual pixels, so text and icon edges look smoother. Smartphones are typically 400+ PPI, while desktop monitors are commonly in the 90–110 PPI range.
The inch-to-cm standard
One inch was fixed at exactly 2.54 cm by the 1959 international yard-and-pound agreement. Screen spec sheets that list size in inches use this same fixed ratio when converted to centimeters.
Practical tip. The screen size manufacturers list (in inches) is usually the panel's diagonal excluding the bezel. If your measurement differs, enter the diagonal inches from the spec sheet instead.
Frequently asked questions
How is PPI calculated?
Square the horizontal and vertical resolution, add them, and take the square root to get the diagonal pixel count; divide that by the diagonal size in inches to get PPI. For example, at a 24-inch diagonal with 1920×1080 resolution, the diagonal pixel count is √(1920²+1080²)≈2202.9px, so PPI≈2202.9÷24≈91.8.
Why do phones and monitors have such different PPI?
A smaller screen produces a higher PPI at the same resolution. For example, a 6.1-inch phone at 2532×1170 has a PPI of about 457, while a 24-inch monitor at 1920×1080 has about 91.8 — screen size drives pixel density far more than resolution alone.
How do I convert diagonal inches to cm?
Multiply by the fixed ratio 1 inch = 2.54 cm. A 24-inch screen is 24×2.54=60.96cm, which is exactly what the calculator's "Diagonal length" output shows.
If the resolution is the same, does screen size still change PPI?
Yes. Since diagonal inches is the denominator in the PPI formula, a smaller screen at the same resolution has a higher PPI and a larger screen has a lower one. For example, 1920×1080 gives PPI≈91.8 at 24 inches but only about 81.6 at 27 inches.