Aspect Ratios Explained (16:9, 4:3, 21:9)
An aspect ratio is the shape of a rectangle — the relationship between its width and height, independent of size. It decides whether a video fills your screen or sits between black bars, and whether an image looks right or stretched. Here is how it works.
What an aspect ratio is
An aspect ratio is written as width:height, like 16:9. It describes proportion, not size: a 16:9 image can be 1280x720 or 3840x2160 and still be the same shape. Divide width by height and 16:9 gives about 1.78 — a useful single number for comparing shapes.
The common ones
- 16:9 — modern TVs, monitors and most online video (widescreen)
- 4:3 — older TVs and many documents and slides (nearly square)
- 21:9 — ultrawide monitors and cinematic film
- 1:1 — square, common for social media posts
- 9:16 — vertical video for phones (stories and reels)
Calculating a missing dimension
To keep a shape while resizing, hold the ratio constant. For 16:9, height = width x 9/16. So a 1600-pixel-wide image should be 1600 x 9/16 = 900 pixels tall. Changing one dimension without the other is what stretches faces and squashes logos. The Aspect Ratio Calculator solves for the missing side.
Why letterboxing happens
When content of one ratio plays on a screen of another, something has to give. Rather than stretch the picture, players add black bars — letterboxing (bars top and bottom) for wide film on a less-wide screen, or pillarboxing (bars at the sides) for a squarer video on a widescreen. The bars preserve the original shape so nothing looks distorted.
Frequently asked questions
Does aspect ratio affect quality? No — it is about shape; resolution (the pixel count) affects sharpness.
Why is my video pillarboxed? Because it is squarer than the screen, so bars fill the sides to avoid stretching it.
What ratio should I record for phones? 9:16 vertical for stories and reels; 16:9 for standard landscape video.
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Last updated: August 27, 2026