Aspect Ratio Calculator
Find the aspect ratio of any width × height, and resize dimensions while keeping the proportion — perfect for video, images and screens.
Resize keeping this ratio
How it works
The aspect ratio is the proportional relationship between width and height, reduced to lowest terms. Enter any dimensions and the calculator divides both by their greatest common divisor: 1920 × 1080 → GCD 120 → 16:9. The decimal form (1.7778) is shown too, since some specs express ratios that way.
The second block solves the everyday follow-up: resizing without distortion. Type a new width and the matching height appears (or vice versa), computed by simple proportion — so a 16:9 frame resized to 1280 wide must be 720 tall.
When to use
Editing video (16:9 for YouTube and TV, 9:16 for Reels/TikTok/Shorts, 21:9 cinematic); preparing
images for social media (1:1 and 4:5 for Instagram feed); designing responsive layouts and
choosing CSS aspect-ratio values; buying a monitor or projector; or scaling artwork and
photos for print without stretching.
The resize block is the practical half: any time a platform demands "at least 1080 px wide" you can instantly get the height that keeps your original proportion.
Practical examples
Full HD video
1920 × 1080 reduces to 16:9. Resizing to 1280 wide gives 720 tall — which is exactly the 720p resolution.
Instagram portrait post
1080 × 1350 reduces to 4:5, Instagram's portrait feed ratio. A photo 2400 px wide would need to be 3000 px tall to match.
Common mistakes
Confusing aspect ratio with resolution: 1920×1080 and 1280×720 are different resolutions with the same ratio (16:9). Content moves between them cleanly; problems (letterboxing, cropping, stretching) appear only when the ratios differ.
The other classic is resizing both dimensions independently "to fit" — that changes the ratio and distorts the image. Change one dimension, compute the other, and crop if the target frame truly needs a different ratio.
Frequently asked questions
What are the most common aspect ratios?
16:9 (TV, YouTube, most screens), 9:16 (vertical video: Reels, TikTok, Shorts), 4:3 (older TV, iPad), 1:1 (square posts), 4:5 (Instagram portrait), 21:9 (ultrawide/cinema) and 3:2 (many cameras and laptops).
How do I fit 16:9 video into a 1:1 frame?
You can’t without a trade-off: either crop the sides (losing content), letterbox with bars above and below, or blur-fill the background. Pick based on where the important content sits.
Why does the tool show a decimal like 1.7778?
Some specifications express ratios as a single number (width ÷ height). 16:9 = 1.7778, 4:3 = 1.3333, 21:9 ≈ 2.3333. It also helps compare ratios that don’t reduce to tidy integers.
What ratio should a YouTube thumbnail be?
YouTube recommends 16:9, ideally 1280 × 720 px. The calculator confirms any custom size you design keeps that ratio.