# Pixels per Inch. Per inch?

The physical properties of a screen, that is the number of pixels are not measured on a surface but as a line. Pixels per Inch (PPI)

For years I tried to figure out screen resolution in relation to the physical properties of a screen. Only now I understood that the physical properties of a screen, that is the number of pixels are not measured on a surface but as a line. So when it says 300 pixels per inch these are on a line an inch long. And there are many more on a square inch. There are 90.000 physical pixels on a square inch of such a screen. Of course assuming the pixels are squares.

A bit one dimensional to mix up lines and surfaces when defining resolution 😉

### Calculations

To have this figured out made me happy. This new knowledge gave me insight and input for calculations.

The PPI of a display can be derived from the diagonal size of a screen in inches and the resolution (horizontal and vertical) in pixels. The calculation is done in two steps.

1. Calculate the diagonal resolution in pixels, relying on the Pythagorean Theorem:

dₒ = √(w² + h²)

w = width and h = height
2. Calculation of the PPI is the total numbers of pixels divided by the diagonal size:

PPI = dₒ/dᵢ

With these two simple formulas I can plug in the numbers.