How to calculate the DPI of the screen with a resolution or diagonal?

DriverMSG wrote:

You will not calculate these parameters. You need to know the size of the displayed spot on the screen.

You need to know the 3 things, resolution, diagonal and screen format, like 16: 9, although the latter results directly from the resolution.

For example, 27 "16: 9, Full HD, the dimensions of the screen from trigonometry from primary school is 13.2371" at 23.5325 ".
1920 / 23.5325 = about 82 ppi
1080 / 13.2371 = around 82 ppi

Of course, 27 inches given by the manufacturer is not a physical dimension given with high accuracy.

PS. The spot size from the above 25.4 / 82 = 0.31mm