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You might have come across the term "aspect ratio" while shopping for a new computer monitor or looking up a film on IMDb. The term is used extensively throughout the video and photography world. At its core, aspect ratio is a simple mathematical concept.
Aspect Ratio in Video and Photography
Aspect ratio is the ratio of the width and the height of a four-sided shape like a TV screen or a photograph. Like any mathematical ratio, the aspect ratio of a rectangle doesn't refer to its physical dimensions, but rather, how the width and length of an object relate to one another.
A perfect square would have an aspect ratio of 1:1 because the width of a square must equal its length. Aspect ratios can be expressed as whole numbers (like 3:2) or with decimal places (like 1.5:1).
Using the aspect ratio, you can work out the height of an object using its width (and vice versa). Probably the most commonly quoted aspect ratio is 16:9, which is often interpreted as "sixteen by nine," where the first number relates to the width (16) and the second to the height (9).
In film production, the aspect ratio refers to the shape of the frame. Two of the most common aspect ratios that you'll see in cinema are 1.85:1 (widescreen) and 2.39:1. This is why you will see black bars on the top and bottom of the frame when watching most movies.
Television has settled on 16:9 for most broadcast content, although displays exist in all sorts of aspect ratios. Ultrawide monitors favored by many gamers may come in an aspect ratio of 2.37:1 (commonly marketed as "21:9"), with so-called "32:9" ultrawide monitors now available from a few manufacturers.
Using an Aspect Ratio Calculator
If you want to fit a specific piece of content to a particular device---for example, a background wallpaper on a smartphone---an aspect ratio calculator like this one by Andrew Hedges can help.
Using this tool, you can calculate height or width based on an aspect ratio, or even work in reverse to work out the aspect ratio of a rectangle based on physical dimensions.
So, for example, to work out the height based on the width and the aspect ratio, enter your aspect ratio into the W1 and H1 boxes, and then add the physical width into the W2 box. The calculator will display the relative height in the H2 field.
You can work out what the aspect ratio of any given rectangle is by simply entering the physical dimensions into the W1 and H1 boxes. The ratio will be displayed above the "Example" image below.
Preserve Aspect Ratio at All Costs
When it comes to content like images and videos, deviating from the aspect ratio when resizing will result in an image looking stretched or squeezed. For best results, always preserve the aspect ratio you're working with to avoid unsightly warping.
Disney famously made this mistake when it decided to stretch classic episodes of The Simpsons on Disney+ to conform with modern 16:9 displays. Fortunately, you can still watch The Simpsons in its original 4:3 aspect ratio with a tweak.