Aspect ratio calculator

Scaling one dimension without the other is what stretches faces sideways. This gives you the number that does not.

What an aspect ratio is

Aspect ratio is width divided by height, written as two numbers with a colon. 16:9 means 16 units across for every 9 down, which is a decimal ratio of about 1.778. Any resolution with that ratio — 1920×1080, 1280×720, 3840×2160 — is 16:9.

Keeping the ratio fixed is what stops an image distorting. Scale width by 1.5 and you must scale height by 1.5 too; change only one and circles become ovals.

Solving for the missing side

The arithmetic is one division. Given a ratio and one dimension:

height = width ÷ ratio
width = height × ratio

For a 16:9 image 1600 px wide: 1600 ÷ 1.778 = 900 px tall. Leave one field blank above and the calculator solves for it.

Simplifying a resolution

To find the ratio of an existing image, divide both dimensions by their greatest common divisor. For 1920×1080 the GCD is 120, giving 16:9. For 2560×1600 the GCD is 320, giving 8:5 — which is more commonly written 16:10.

Some resolutions do not simplify to anything recognisable. 1366×768, once ubiquitous on budget laptops, is 683:384 — very close to 16:9 but not exactly, which is why full-screen 16:9 video on those panels was always very slightly scaled.

The ratios worth knowing

16:9 (1.778) — the default for video, streaming and most monitors.
4:3 (1.333) — pre-HDTV broadcast, and still used by many tablets and projectors.
3:2 (1.5) — the 35 mm film frame, inherited by most DSLR and mirrorless cameras.
1:1 — square, popularised by early Instagram.
9:16 (0.5625) — vertical video for phones: stories, Reels, TikTok.
21:9 and 2.39:1 — ultrawide monitors and anamorphic cinema respectively. Not the same ratio, despite often being conflated.

Even numbers matter for video

Most video codecs, H.264 included, encode in macroblocks and require even pixel dimensions — many require multiples of 8 or 16 for the best results. An odd height like 1081 will be rejected or silently padded by most encoders.

The calculator shows the nearest even dimensions for this reason. When a proportional resize lands on an odd number, round to even rather than accepting the exact ratio; a half-pixel of distortion is invisible, a failed encode is not.

Letterboxing and cropping

When the source and target ratios differ, something has to give. Letterboxing adds bars to preserve the whole frame; cropping fills the frame but loses content from the edges. Stretching does neither honestly, and is always the wrong answer.

This is the everyday problem in social media video: a 16:9 landscape clip placed in a 9:16 vertical slot either gets thick bars top and bottom, or loses roughly two-thirds of its width.

Common questions

How do I calculate a missing dimension?

Divide the ratio out. Height equals width divided by the decimal ratio, and width equals height times it. For 16:9 the decimal ratio is 1.778, so a 1600 px wide image is 900 px tall.

What resolution is 16:9?

Any where width divided by height is about 1.778 — 1920x1080, 1280x720, 3840x2160 and 2560x1440 are all 16:9. The ratio describes the shape, not a specific pixel count.

Why do video dimensions need to be even?

Codecs such as H.264 encode in macroblocks and require even dimensions, often multiples of 8 or 16. An odd height will be rejected or silently padded, so round to even after a proportional resize.

Is 21:9 the same as cinema widescreen?

Not quite. Ultrawide monitors marketed as 21:9 are usually 2.33:1, while anamorphic cinema is 2.39:1. Close, but a true cinema frame still letterboxes slightly on a 21:9 display.