Aspect ratio calculator
Enter a width and height to see their ratio in lowest terms, or pick a ratio and one known side to get the other one. Useful for resizing images, video, screens and layouts without stretching them.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Choose Find the ratio to reduce a width and height, or Find a missing side to resize.
- For a ratio, type the width and height. The simplified ratio appears at once.
- For a missing side, choose whether you need the height or the width, enter the side you know and pick a preset ratio or a custom one.
How it works
Two numbers have the same ratio when dividing both by the same number gives the same pair. The calculator divides by the greatest common divisor, found with Euclid’s algorithm, so 1920:1080 becomes 16:9. To find a missing side it keeps the ratio fixed:
- height = width × ratio height ÷ ratio width
- width = height × ratio width ÷ ratio height
Worked examples
- 1024 × 768 has a divisor of 256 and reduces to 4:3.
- 16:9 at 1280 wide gives a height of 1280 × 9 ÷ 16 = 720, so 1280 × 720.
- 9:16 at 1080 tall gives a width of 1080 × 9 ÷ 16 = 607.5, which you would round to a whole pixel size.
Tips
- Resize by keeping the ratio and changing only one side; changing both freely stretches the picture.
- Video and image editors need whole pixels, so round the result and check that the ratio is still close enough, or crop a few pixels.
- Portrait ratios are the landscape ones turned around: 9:16 is 16:9 rotated.
Frequently asked questions
How do I find the aspect ratio of an image or screen?
Divide width and height by their greatest common divisor. For 1920 × 1080 the divisor is 120, so the ratio is 16:9. Choose "Find the ratio", enter both sizes and the result is shown in lowest terms.
How do I calculate the height for a given width?
Multiply the width by the ratio's height part and divide by its width part. At 16:9 and a width of 1280 the height is 1280 × 9 ÷ 16 = 720. Choose "Find a missing side", pick the ratio and enter the width.
Why does the ratio not match a standard one like 16:9?
Many real sizes are only close to a standard ratio, for example 1366 × 768 reduces to 683:384, which is nearly 16:9. The decimal value (width ÷ height) helps you compare it with 1.7778 for 16:9 or 1.3333 for 4:3.
Can I use decimals?
Yes. Values such as 2.5 × 2 are scaled to whole numbers (25 × 20) before being reduced, so the ratio comes out as 5:4. Up to six decimal places are handled exactly.