16:9 · 4:5 · 9:16
Aspect ratio calculator
Enter your original dimensions and one new dimension. The calculator returns the other, so nothing gets stretched.
Calculated in your browser · nothing is sent anywhere
Why proportions break
Every video has a fixed ratio between width and height. Change one without changing the other in step and the picture stretches — faces widen, circles turn into ovals. It is immediately obvious to a viewer even when they cannot say what is wrong.
The arithmetic is simple but easy to get wrong under time pressure, especially with unusual source dimensions. A screen recording of a laptop display might be 2880 by 1800, which is 16:10 rather than 16:9. A clip cropped in a phone app can end up at some ratio nobody planned.
The tool also flags a detail most calculators skip: H.264 requires both dimensions to be even numbers. An odd result will be silently adjusted by your encoder, sometimes shifting the framing by a pixel and occasionally producing a green line down one edge.
Both fields update each other. Type a width and the height follows; clear it and type a height instead and the width recalculates. The source ratio is reduced to its simplest form so you can see at a glance whether you are working with 16:9, 4:3 or something less standard.
One thing worth understanding: this calculator scales, it does not reshape. Turning 16:9 footage into a 1:1 square is not a resize at all — it requires either cropping the sides away or adding bars, and that is a decision about what the viewer sees rather than arithmetic.
The ratios worth knowing
16:9 covers almost all landscape video, from YouTube to television. 9:16 is vertical for Reels, Shorts and TikTok. 1:1 square still appears in feed posts, and 4:5 is the tallest shape Instagram allows in the main feed. Cinema uses 2.39:1, which is why letterboxing appears on films.
To actually apply new dimensions to a file, use change resolution.
Common questions
What does 16:9 actually mean?
For every 16 units of width there are 9 of height. 1920×1080 and 1280×720 are both 16:9, which is why one scales cleanly to the other.
Why must dimensions be even?
H.264 encodes colour information in 2×2 pixel blocks, so odd dimensions cannot be divided cleanly. Encoders adjust silently, which can shift your framing by a pixel.
Can I change the ratio without stretching?
Only by cropping or adding bars. There is no way to change shape while keeping every pixel undistorted — something has to be cut off or filled in.
What ratio is my screen recording?
Often 16:10 rather than 16:9, because many laptop displays are 16:10. Enter the recorded dimensions and the calculator will show you the reduced ratio.
Why does my video have black bars after resizing?
The target shape did not match the source ratio, so the player added bars to avoid distorting the image. That is the correct behaviour.
Is anything uploaded?
No. This is arithmetic running in your browser and it touches no network.