If you don't know what ratio your screen is, you just take the resolution and divide the larger number by the smaller. 1024/768=1.33 Here's the summary:
5:4, 1280x1024 = 1.25, Non-widescreen LCD monitors.
4:3, 800x600, 1024x768 = 1.33, Non-widescreen TVs and monitors
16:10, 1280x800, 1680x1050, 1920x1200 = 1.6, Widescreen monitors
16:9, 1280x720, 1600x900, 1920x1080, 2560x1440 = 1.78, Widescreen TVs and newer monitors
Type a new resolution here to add it to the list: