Formula
The pixel diagonal comes from the Pythagorean theorem, then it is divided by the physical diagonal:
PPI = sqrt(width² + height²) / inches
The result is rounded to one decimal. Print DPI is a different decision: it turns pixels into a paper size and lives on the print-size page.
Assumptions
- The diagonal is the visible screen size in inches, not the pixel diagonal.
- Pixels are square.
- The whole resolution is used, with no scaling or black bars.
Worked example
A 1920 by 1080 screen at 24 inches is 91.8 PPI.
Input definitions
- Horizontal and vertical pixels
- The resolution, such as 1920 by 1080.
- Diagonal
- The advertised screen size in inches.
Limitations
Viewing distance changes whether a density looks sharp. This page does not recommend a distance or a print size.
Questions
Is PPI the same as print DPI?
The words are related, but the questions differ. PPI here describes a screen. Print size divides pixels by a chosen DPI to get inches of paper.
Does this include the bezel?
No. Use the viewable diagonal, which is the size manufacturers advertise.