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Guide
About the Aspect Ratio Calculator
An aspect ratio is the proportion between a shape’s width and height, written as two numbers such as 16:9. Two sizes with the same ratio — 1280×720 and 1920×1080 — look identical when scaled, while changing the ratio stretches, squashes or crops the picture. This calculator tells you the ratio of any size and the matching dimensions at any other size.
How to calculate an aspect ratio
Divide the width and height by their greatest common divisor. For 1920×1080 that divisor is 120, giving 16:9. To resize, multiply: a 16:9 image 1280 pixels wide is 1280 × 9 ÷ 16 = 720 pixels tall. Many screen sizes do not reduce neatly — 1366×768 is 683:384 — so the calculator also shows the nearest familiar ratio, which here is 16:9 at 0.05% off.
Common aspect ratios
- 16:9 — HD and 4K video, YouTube, most monitors and TVs.
- 9:16 — vertical video for Stories, Reels, TikTok and Shorts.
- 4:3 — older TVs, iPads and presentation slides.
- 1:1 and 4:5 — square and portrait social posts.
- 1.91:1 — Open Graph link previews (1200×630).
- 21:9 — ultrawide monitors and cinematic video.
How to use it
- 1Enter the original width and height, or pick a preset ratio.
- 2Read the simplified ratio and the closest common one.
- 3Type a new width or height to get the other side.
- 4Copy the new size or the CSS.
Frequently asked questions
What aspect ratio is 1920×1080?
16:9. Both numbers divide by 120, leaving 16 and 9.
What size is 16:9 at 1280 wide?
1280×720. Multiply the width by 9 and divide by 16.
Why round to even numbers?
H.264 and most other video codecs work on 2×2 blocks of pixels and reject odd widths or heights, so video sizes should be even.
How do I keep an aspect ratio in CSS?
Use the aspect-ratio property, for example aspect-ratio: 16 / 9, together with a width. The element’s height then follows automatically.
Is 1366×768 16:9?
Almost. It reduces to 683:384, which is within 0.1% of 16:9, so it is treated as 16:9 in practice.