Formula
The angular width comes from the seating distance. Pixels divide that angle:
PPD = pixels / (2 × atan(width / 2 / distance))
The angle is converted from radians to degrees. The screen PPI page divides a pixel diagonal by inches and never asks where you sit.
Assumptions
- Width and distance use the same inch.
- You face the centre of the screen.
- Only the horizontal pixels are counted.
Worked example
2,560 pixels across 23.5 inches, viewed from 24 inches, is 49.1 PPD.
Input definitions
- Image width
- The horizontal size of the lit area, not the diagonal.
- Pixels per degree
- How many horizontal pixels sit inside one degree of your view.
Limitations
This is a planning figure from the numbers you type. It is not a measurement of a network, a camera, a printer, or a power supply.
Questions
Why not PPI?
PPI ignores distance. Two screens with the same PPI look different if you sit at different distances. This page includes the distance.
What width is a 27-inch 16:9 monitor?
About 23.5 inches across the image. The example uses that width rather than the 27-inch diagonal.