Free Aspect Ratio Calculator
Enter a ratio and one side to get the other, or reduce any width and height to its simplest ratio. Results update as you type.
Enter values above zero for the ratio and the size.
What is an Aspect Ratio Calculator?
An aspect ratio is the proportion of width to height, written like 16:9. An aspect ratio calculator keeps that proportion while you resize, so images and video do not stretch.
Change the width and the height follows, or the other way around. To work out the ratio of an existing file, untick Lock ratio and enter its size.
- Enter a ratio, or choose a preset such as 16:9.
- Type a width to get the height, or a height to get the width.
- To find an unknown ratio, untick Lock ratio and enter both sides.
Why Calculate It?
- No stretching: keeping the ratio is what stops a resized image from looking squashed.
- Exact figures: you get the number to put in a design file, a CSS rule or an export setting.
- See it first: the preview shows the shape before you commit to a crop.
Common Aspect Ratios
The ratios you will meet most often, and where they are used.
| Ratio | Example size | Where it is used |
|---|---|---|
| 1:1 | 1080 × 1080 | Square. Profile images and social feed posts. |
| 4:3 | 1024 × 768 | Older televisions and monitors, tablets, and many compact cameras. |
| 3:2 | 1500 × 1000 | The classic 35mm photo shape, shared by many DSLR and mirrorless cameras. |
| 16:9 | 1920 × 1080 | The standard for HD video, monitors, and most online video. |
| 21:9 | 2560 × 1080 | The marketing name for ultrawide monitors and a cinematic look. Those screens are not exactly 21:9, which is why this size reduces to 64:27. |
| 9:16 | 1080 × 1920 | Vertical video for stories, reels and short-form feeds. |
| 4:5 | 1080 × 1350 | Portrait social posts, which take up more of a phone screen than a square. |
How to Read the Result
Four figures describe the same shape.
The ratio
SimplifiedYour ratio in lowest terms, reduced by the greatest common divisor. 1920 by 1080 becomes 16:9.
Width divided by height
DecimalThe same ratio as one number, which is handy for comparing shapes. 16:9 is 1.778, and anything above 1 is landscape.
The missing side
SolvedHeight is width times the ratio height, divided by the ratio width. Results are rounded to two decimals, so round to whole pixels for an export.
The shape
PreviewA scaled box showing the proportions. It shows the shape only, not the actual size.
What an Aspect Ratio Tells You
A ratio describes shape and says nothing about size. A 640 by 360 clip and a 3840 by 2160 clip are both 16:9, so they fill the same frame at very different resolutions.
When source and frame differ, something has to give: the image is cropped, letterboxed with bars, or stretched. Knowing the ratio lets you choose.
The Math Behind It
To simplify a ratio, divide both sides by their greatest common divisor. To resize, multiply the new width by height over width from the original.
For a 16:9 frame:
- Width 1280: 1280 × 9 ÷ 16 = 720 high.
- Height 1080: 1080 × 16 ÷ 9 = 1920 wide.
Where Aspect Ratios Trip People Up
The arithmetic is easy. These are the cases that still catch people out:
- Rounding drift: some sizes cannot hit a ratio exactly in whole pixels, so 1366 by 768 is only about 16:9.
- Odd numbers in video: many encoders need even dimensions, so round to the nearest even pixel.
- Reserve the space in CSS: the aspect-ratio property, or width and height attributes on an image, stops the page jumping as media loads.
- Platforms change their specs: check the current size guidance before a campaign, since these numbers move.
- Matching colors as well as shapes? The hex to RGB converter gives you every format a design tool or stylesheet needs.
Why the Ratio Matters
Clean layouts
Images that share a ratio line up in a grid without awkward gaps.
Controlled crops
Planning the ratio decides what gets cut, and leaves nothing to an automatic crop.
Stable pages
Declaring dimensions prevents layout shift, a measure in Core Web Vitals.
Right for each channel
One asset rarely fits every feed, so each channel needs its own ratio.
Who Uses an Aspect Ratio Calculator
Anyone resizing something that has to keep its shape.
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See domain monitoringFrequently Asked Questions
Common questions about aspect ratios.
How do I calculate an aspect ratio?
Divide the width and height by their greatest common divisor. For 1920 by 1080 that divisor is 120, which gives 16:9.
What is the aspect ratio of 1920x1080?
16:9, the standard for full HD video and most monitors.
What aspect ratio is 4K?
Consumer 4K UHD is 3840 by 2160, which is 16:9. Cinema 4K is 4096 by 2160, which is close to 1.9:1.
How do I find the height from a width?
Multiply the width by the ratio height, then divide by the ratio width. A width of 1280 at 16:9 gives 720.
Does resolution change the aspect ratio?
No. A ratio describes shape, so many resolutions share one ratio.
Can I use decimals?
Yes. A cinema ratio such as 2.35:1 works, and is reduced to whole numbers for display.
What aspect ratio should I use for video?
16:9 for landscape players and 9:16 for vertical short-form feeds.
How do I set an aspect ratio in CSS?
Use the aspect-ratio property, for example aspect-ratio: 16 / 9, and set a width. The browser works out the height.
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