Aspect ratios are mathematically expressed as x :y (pronounced "x-to-y") and x×y (pronounced "x-by-y"). The most common aspect ratios used today in the presentation of films in movie theatres are 1.85:1 and 2.39:1. The aspect ratio of an image is its width divided by its height.
Here is a simple free utility which lets you find out the aspect ratio of your current monitor resolution.
The program calculates your computer’s current aspect ratio and displays it in the most common format. It works on the following operating systems: Windows 95, 98, ME, NT, 2000, XP, Vista, 7. (Win32)