# Pixels per Inch. Per inch?

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 ðŸ˜‰

### 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.

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

dâ‚’ = âˆš(wÂ² + hÂ²)

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

PPI = dâ‚’/dáµ¢

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

Outcome for my ereader

So for the screen of six inches diagonal and resolution of Â 1072 by 1448 it is:

dâ‚’ = 1801.6

It is six inches and so I divide by six.

PPI = 300.27.

Close! Pretty, pretty close. And now the theory is clear I also can use and understand this calculator: