Aspect Ratio Calculator
Fill in one new dimension and the other is worked out.
Simplifying, and scaling without distortion
An aspect ratio is width divided by height, reduced by the greatest common divisor. 1920 × 1080 share a divisor of 120, giving 16:9.
Fill in either new dimension above and the other is calculated to preserve the shape. That is the everyday use: resizing an image or a video frame without squashing it.
Ratios worth recognising
- 16:9 — the default for televisions, most monitors and video. Chosen in the 1980s as a compromise between 4:3 television and cinema's wider formats: it is the geometric mean of the two.
- 4:3 — old televisions and monitors, and still used by some cameras and projectors.
- 3:2 — 35 mm film and most system cameras. Prints at 6×4 inches with nothing cropped.
- 21:9 — ultrawide monitors, approximating cinema shape. Usually actually 64:27 or 43:18, since a true 21:9 does not divide into common resolutions.
- 9:16 — vertical video, now the dominant format for phone-first platforms.
Where mismatched ratios show up
Put 4:3 content on a 16:9 screen and something has to give:
- Pillarboxing — black bars at the sides, preserving the whole image. The honest option.
- Cropping — filling the screen by cutting the top and bottom off.
- Stretching — filling the screen by distorting it. Everyone becomes slightly wider, and once you notice a television doing this you cannot stop noticing.
Letterboxing is the same problem in reverse: cinema content at 2.39:1 on a 16:9 television leaves bars top and bottom.
Ratios do not tell you about size
16:9 describes shape only. A phone screen and a cinema screen can share it. For the physical dimensions you need a diagonal as well, which is what the screen size calculator is for.
Common uses
- Resizing an image without distorting it.
- Working out a video export size.
- Setting up a CSS aspect-ratio box.
- Checking what shape a resolution is.
Questions people ask
How do I calculate an aspect ratio?
Divide both dimensions by their greatest common divisor. 1920 × 1080 share 120, giving 16:9.
Why is 16:9 the standard?
It was chosen in the 1980s as a compromise between 4:3 television and wider cinema formats — it is the geometric mean of the two.
What is 21:9 really?
Usually 64:27 or 43:18. A true 21:9 does not divide neatly into common resolutions, so the label is approximate.
How do I resize an image without distorting it?
Set one new dimension and let the other follow from the ratio. That is what the second row above does.
What is the difference between letterboxing and pillarboxing?
Letterboxing puts bars top and bottom, for content wider than the screen. Pillarboxing puts them at the sides, for content narrower than it.
Nothing is sent anywhere
The arithmetic runs in the page. Where a tool reads your screen dimensions, it reads what the browser already exposes to every site you visit — and it does not transmit them. The about page lists every request this site makes.