Aspect Ratio Calculator
Change a ratio or a dimension and the other side recalculates immediately, with a live preview of the shape.
An aspect ratio is the relationship between width and height. Getting one wrong is how images end up stretched, videos end up letterboxed and layouts end up shifting as they load.
How to use it
- 1Pick a preset ratio or enter your own.
- 2Type a width or a height, the other side solves automatically.
- 3Check the preview to confirm the shape is what you expected.
The ratios worth memorising
16:9 is the standard for video and most screens, 1920×1080, 1280×720, 3840×2160 are all the same shape. 4:3 is the old television and camera-sensor ratio, still common in presentations and vintage photography. 1:1 is the square used across social platforms. 3:2 matches most DSLR sensors and standard photographic prints. 21:9 is the ultrawide cinematic shape.
For social specifically: 9:16 is the vertical full-screen format used by Stories, Reels and TikTok, and 4:5 is the tallest ratio Instagram allows in the main feed, which is why it's the most efficient shape for feed posts.
Reserving space to stop layout shift
When an image loads without declared dimensions, the browser doesn't know how tall it will be, so everything below it jumps when it arrives. That jump is measured by Cumulative Layout Shift, one of Google's Core Web Vitals, and it's both a ranking factor and a genuine annoyance.
The fix is to tell the browser the shape in advance. Setting width and height attributes on an img element is enough, modern browsers derive the aspect ratio from them and reserve the space, even when CSS resizes the image responsively. For containers and embedded iframes, the CSS aspect-ratio property does the same job.
Scaling without distortion
To resize an image and keep its shape, multiply both dimensions by the same factor. A 1920×1080 image at half size is 960×540; at a third it's 640×360. If you only know one target dimension, solve for the other, that's exactly what this calculator does.
When you must fit content into a ratio it doesn't match, you have three choices: crop, letterbox, or distort. Crop is almost always the right answer for photography, letterbox for video where the full frame matters, and distortion is never the right answer.
Common questions
- What size is 16:9?
- Any size where width divided by height equals 1.777…. Common examples are 1920×1080, 1600×900, 1280×720 and 640×360.
- How do I work out the missing dimension?
- Multiply the known dimension by the ratio. For 16:9, height = width × 9 ÷ 16. Enter one value above and the calculator does it for you.
- Which ratio should I use for social media?
- 1:1 or 4:5 for feed posts, 9:16 for Stories, Reels and TikTok, and 16:9 for YouTube. 4:5 takes up the most vertical space allowed in an Instagram feed.
- Does aspect ratio affect page speed scores?
- Indirectly but meaningfully. Declaring dimensions or an aspect-ratio lets the browser reserve space, which reduces Cumulative Layout Shift, one of the Core Web Vitals.
- Is 1.85:1 the same as 16:9?
- Close but not identical. 16:9 is 1.78:1. 1.85:1 is a cinema projection standard and will letterbox very slightly on a 16:9 screen.