To calculate screen size:

1. Measure the length and width of the screen in the same units.
2. Multiply both answers to get the screen size — the area of the screen.
3. If you're after the size of the screen's diagonal, you can either measure it or use Pythagoras' theorem where a & b are the screen's length and width.

To calculate the size of a screen's diagonal:

1. Measure the length and width of the screen in question.
2. Square both measurements and add them together.
3. Find the square root of the summand.
4. You've found your diagonal screen size!

The size of a 16:9 screen depends on how long the screen's diagonal is, as 16:9 is merely the ratio of the screen's width to its height. If you have the screens diagonal, you can multiply that measurement by 0.872 to get the screen's width. You can also multiply the diagonal by 0.49 to get the screen's height. Then, if you wish to find the screen area, multiply the width by the height.