Whether you are showing an image on a projection screen or display, you need a way to describe that image's width as compared to its height. Do this with a ratio of width to height called the aspect ratio.

Every image, screen and display has a specific size based on how it will be used. Your ability to determine the size (aspect ratio) of any image, screen or display helps you buy the right product, match a projector's image to the correct screen size, or select the correct image size for a given environment. Without a solid foundation in this area, you may make a costly error.

Glossary

Aspect Ratio

Aspect ratio is the ratio of image width to image height. It can be expressed as two whole numbers separated by a colon, e.g. 16:9, or by the decimal equivalent of the implied division, e.g. 1.77. Common video aspect ratios include the following:

4:3 (1.33) for Standard-Definition (SD) video very common in late 20th century
16:9 (1.77) for High-Definition (HD) video
Some computer and video production displays added space below the 16:9 aspect ratio of HD video and made a 16:10 (1.6) aspect ratio.