Okay, I have a screen which is given a size, but I want to know what size that really is.

Step 1. Figure out what the aspect ratio of your LCD is.

Standard aspect ratios typically come in 2 sets.
  1. 4:3 is popular, and is pretty standard. Native resolutions of 320X240, 640X480, 800X600, 1024X768 etc fall into this category, as do standard TVs.
  2. 5:4 is common for 17" and larger monitors, typically 1280X1024 resolution.

Widescreen monitors will typically come in 3 sets.
  1. 16:9 is the nominal widescreen resolution, but will typically ONLY come for TVs. 720p and 1080p resolutions fit. Johnzo had some 1366X768 monitors that fit this aspect ratio perfectly as well.
  2. 15:9 is typical WXGA (1280X768) though other things fit, too. The eVo, for example fits this at 800X480.
  3. 16:10 is typical of WUXGA (1920X1200) Some monitors are improplerly labeled as WXGA, and are 1280X800. These fit here, too

So, there are some magic numbers that you use to calculate the height and width from the diagonal size.

I'll use 4:3 as an example, since that's the simplest.

First off, we use pythagorean theorum to get a ratio of height and width to diagonal. a^2 + b^2 = c^2.
That gives us 4^2 + 3^2 = c^2
which becomes 16 + 9 = c^2
25 = c^2
5 = c

Okay, so for every 5 units of diagonal, we get 4 units of width, and 3 units of height. we take ratios by dividing the width and height by the diagonal Ž©
Width = 5 / 4 = 0.800
Height = 5 / 3 = 0.600

For a 15" 4:3 LCD:


We take the diagonal measurement, 15" and multiply by the ratio fo the diagonal to the width, which is 0.800, and get 12" wide.
Doing the same, but using the ratio for the height, which is 0.600, we get 9"

I will use X for width and Y for height. Multiply these values by the diagonal size to get the width and height respectively.
  • 4:3 X=0.800, Y=0.600
  • 5:4 X=0.781, Y=0.625
  • 16:9 X=0.872, Y=0.490
  • 15:9 X=0.857, Y=0.514
  • 16:10 X=0.848, Y=0.530


Edit: Whoops. I just realised that this is a duplication of one that I'd done before. I just ran across the same question again, and assumed that it wasn't here.

In any case, since this does have some information concerning resolutions which was not in the old topic, I'll leave this one and remove the older.