About Aspect Ratio Calculator Online (16:9, 4:3)
An aspect ratio describes the proportional relationship between a width and a height — 16:9 for most video, 4:3 for legacy displays, 1:1 for square social posts. When you resize an image or video frame, keeping that ratio fixed is what stops everything from looking stretched or squashed.
This free aspect ratio calculator solves the missing dimension for a target ratio: pick a ratio (or type your own), enter a width or a height, and get the matching value instantly. It also reduces any width × height to lowest terms, so 1920 × 1080 reads back as a clean 16:9.
Everything runs in your browser — there's nothing to upload and no math to do by hand.
Features
- Solve the missing width or height for any target ratio
- Reduce any width × height to its lowest-terms ratio (GCD)
- Common presets: 16:9, 4:3, 1:1, 21:9 — or enter a custom ratio
- Live visual preview at the computed ratio; fully offline
How to use
- Pick a preset ratio or type your own into the two ratio fields.
- Enter the dimension you know — a width or a height.
- Read the solved dimension and the reduced ratio from the result.
- Copy the result, or adjust the ratio to compare sizes live.
Frequently asked questions
What is an aspect ratio?
It is the ratio of width to height, written as W:H. 16:9 means the width is 16 units for every 9 units of height. The same ratio holds at any size, which is why 1280 × 720 and 1920 × 1080 are both 16:9.
How do I keep an image the same ratio when resizing?
Decide the ratio you want, then fix one dimension and let the other follow. Enter your target ratio and the known width (or height), and this calculator returns the matching value so the proportions stay identical.
How is the ratio reduced to lowest terms?
It divides both numbers by their greatest common divisor (GCD). For 1920 × 1080 the GCD is 120, so both sides divide down to 16:9 — the simplest equivalent ratio.
Can the solved dimension be a fraction?
Yes. Not every width maps to a whole-number height for a given ratio, so the result may include a decimal. Round to the nearest pixel if you need an integer size for export.
Does any of this leave my browser?
No. The calculation is plain arithmetic that runs entirely on your device. Nothing you type is sent anywhere.
Related tools
Everything runs locally in your browser — your input is never uploaded.