The proportion of a screen's width to its height is called its aspect ratio. It is expressed as the width, followed by a colon, then the height of the screen in arbitrary units. These numbers are usually expressed as integers (e.g. 4:3), or normalized so that the height is 1 (e.g. 1.33:1). The aspect ratio describes the shape of the screen and is independent from its size. That is, a 13" television and a 27" television may have the same aspect ratio, even though the latter is twice as large.

Traditional computer and television screens have an aspect ratio of 4:3 (or 1.33:1). Widescreen and HDTV screens have an aspect ratio of 16:9 (or 1.77:1). Most movies in the theater and on DVD are at 2.35:1.

Examples of common aspect ratios. Each rectangle has approximately the same area.

On computers pixels are usually square but can sometimes be rectangular, in which case the pixels have an aspect ratio as well.