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Aspect Ratio Calculator
Enter the width and height of an image, video or screen to reduce them to a simple ratio such as 16:9. Or give a new width or height and get the matching size that keeps the original proportions.
Reducing a size to a ratio
An aspect ratio is the width-to-height relationship written in lowest terms. To reduce it, divide both numbers by their greatest common divisor (GCD). For 1920 × 1080 the GCD is 120, so the ratio is 16:9. For 1280 × 1024 the GCD is 256, giving 5:4.
ratio = width ÷ GCD : height ÷ GCD
Some sizes do not reduce to a tidy pair. A 1366 × 768 screen reduces only to 683:384, but its decimal value of 1.7787 is within 0.05% of 16:9, so it is sold as a 16:9 display. The calculator therefore also names the closest common ratio and states how far off it is.
Resizing without distortion
To keep proportions, the new width and height must keep the same ratio as the original. The cross-multiplication is:
new height = new width × original height ÷ original width
Scaling 1920 × 1080 to a width of 1280 gives 1280 × 1080 ÷ 1920 = 720. If the answer is not a whole number, round it to the nearest pixel; the slight error is invisible. Many video encoders also require even width and height values, so choose a size that satisfies that. For a social media upload, work from the platform's recommended frame and crop your original to its ratio first, then resize.
Stretching an image to a new width and height that do not share the original ratio makes circles into ovals. Use scaling when you must keep every pixel, and cropping when the target frame has a different shape, accepting that some of the edges are lost.
Common ratios and where they appear
| Ratio | Typical use |
|---|---|
| 16:9 | Most video, laptop and monitor screens |
| 4:3 | Older TV and some tablets and compact cameras |
| 3:2 | Many cameras and photo prints (6 × 4 inches) |
| 1:1 | Square social posts, profile pictures |
| 4:5 and 9:16 | Portrait feed posts and vertical video |
| 21:9 | Ultrawide monitors and cinematic formats (approximate) |
Platforms set their own recommended sizes, and those change over time, so confirm the current requirement on the platform you are targeting.
Limits
Ratios are shown for the pixel values you enter. Screens with non-square pixels and some professional video formats define their display ratio separately from the pixel count. Decimals up to three places are accepted, which is useful for physical sizes such as a 4.5 by 3 centimetre print. If you also need the physical sharpness of a screen of a given size, see the screen PPI calculator.
Worked example
A 1920 × 1080 video is scaled down to 1280 pixels wide. What height keeps the picture undistorted?
Inputs
| What do you want to do? | Find the height for a new width |
|---|---|
| Original width | 1920 px |
| Original height | 1080 px |
| New width | 1280 px |
Result
New size 1,280 × 720. Height = new width × height ÷ width.
| Reduced ratio | 16:9 |
|---|---|
| Decimal ratio (width ÷ height) | 1.7778 |
| Closest common ratio | 16:9 |
| Rounded to whole pixels | 1280 × 720 |
Heights for common widths at this ratio
| Width (px) | Height (px) |
|---|---|
| 320 | 180 |
| 640 | 360 |
| 800 | 450 |
| 1024 | 576 |
| 1280 | 720 |
| 1920 | 1,080 |
| 2560 | 1,440 |
| 3840 | 2,160 |
Accuracy note: Results are calculated in your browser from the values you enter. Check anything that matters for legal, academic or financial purposes against the official source.
Frequently asked questions
How do I calculate an aspect ratio?
Divide the width and the height by their greatest common divisor. For 1920 by 1080 the divisor is 120, which gives 16 by 9, so the ratio is 16:9.
How do I resize an image and keep its proportions?
Multiply the new width by the original height and divide by the original width to get the new height. The calculator does this and also works the other way round.
Why does 1366 by 768 not simplify to 16:9?
Its exact reduced ratio is 683:384, which is extremely close to 16:9 but not equal. Manufacturers round it to 16:9, and the tool shows the closest common ratio with the difference.
What is the aspect ratio of a 4K screen?
A 3840 by 2160 screen is 16:9, the same as full HD. Only the pixel count differs, with four times as many pixels in total.
Should I round the new height?
Yes, round to the nearest whole pixel. If you are exporting video, prefer a size where both width and height are even numbers, since many encoders require it.