Keep images and video in proportion
Every screen, photo and video frame has a shape, and the aspect ratio describes it: width compared with height. A 1920 by 1080 video is 16:9, the shape of most TVs and monitors. A phone video held upright is 9:16. Get the ratio wrong when you resize and people look squashed or stretched.
This calculator does both everyday jobs. Give it a width and height in pixels and it simplifies them to a ratio and tells you what that shape is usually used for. Or give it a ratio and one side, and it works out the other side so your resize stays in proportion.
How to use it
- Choose Find the ratio if you have a size, or Resize to a ratio if you have a ratio and one side.
- For a ratio, type the width and height in pixels.
- For a resize, enter the ratio (like 16 and 9), choose whether you know the width or the height, and type that size.
- Read the ratio or the missing side. A resize also shows the size rounded to whole pixels, which is what image tools need.
The maths behind aspect ratios
ratio = (width ÷ GCD) : (height ÷ GCD)
new height = new width × ratio height ÷ ratio width
new width = new height × ratio width ÷ ratio heightGCD is the greatest common divisor: the largest whole number that divides both sides exactly. Dividing both sides by it gives the simplest whole-number ratio. For a resize, the ratio acts like a recipe: keep the two sides in the same proportion and the picture keeps its shape.
Worked example: 1920 by 1080
- The largest number that divides both 1920 and 1080 is 120
- 1920 ÷ 120 = 16 and 1080 ÷ 120 = 9
- So the ratio is 16:9, or 1.78:1 as a decimal
Now say you need the same shape at 1280 pixels wide. Choose Resize to a ratio, enter 16 and 9, pick Width and type 1280. The height is 1280 × 9 ÷ 16 = 720, so the new size is 1280 by 720.
Common aspect ratios and sizes
| Ratio | Used for | Common sizes |
|---|---|---|
| 16:9 | TVs, monitors, online video | 1280 × 720, 1920 × 1080, 3840 × 2160 |
| 9:16 | Phone video, stories, reels | 1080 × 1920 |
| 4:3 | Older screens, many tablets | 1024 × 768, 1600 × 1200 |
| 3:2 | Many cameras and laptops | 3000 × 2000, 6000 × 4000 |
| 1:1 | Square social posts | 1080 × 1080 |
| 4:5 | Portrait social posts | 1080 × 1350 |
| 21:9 | Ultrawide monitors | 2560 × 1080, 3440 × 1440 |
Some screens are marketed as 21:9 but measure a little differently, like 2560 by 1080, which simplifies to 64:27. The calculator shows the exact ratio and the nearest common one so you can tell how close they are.
Resizing mistakes to avoid
- Typing a new width and height that do not match the ratio. The image gets stretched. Change one side and let the other follow.
- Rounding too early. Work out the exact missing side, then round once to a whole pixel at the end.
- Mixing up landscape and portrait. 16:9 and 9:16 are the same shape turned on its side. The width always comes first.
- Confusing ratio with resolution. 1280 by 720 and 3840 by 2160 share a 16:9 ratio but have very different detail. Pixel density on a real screen is a separate question for the PPI calculator.
Frequently asked questions
How do I calculate an aspect ratio?
Find the largest number that divides both the width and the height exactly, then divide both by it. For 1280 by 720, that number is 80, so the ratio is 16:9. Enter the two sizes above and the calculator simplifies them, shows the decimal form, and names the closest common ratio.
What is the height for a 16:9 image that is 1080 wide?
Multiply the width by 9 and divide by 16: 1080 × 9 ÷ 16 = 607.5, so about 608 pixels. For a full 16:9 frame in that width you would usually pick a size like 1080 by 608 or switch to a standard size such as 1920 by 1080. Choose Resize to a ratio to try other widths.
What aspect ratio is 1920 by 1080?
It is 16:9. Both numbers divide by 120, giving 16 and 9. That is the standard shape for full HD video, most TVs and most computer monitors. 1280 by 720 and 3840 by 2160 are also 16:9, just with fewer or more pixels.
What is the difference between aspect ratio and resolution?
Aspect ratio is the shape: how wide the picture is compared with how tall. Resolution is the number of pixels, like 1920 by 1080. Two images can share a ratio and have very different resolutions. A resolution calculator usually needs both: the ratio fixes the shape, the pixel count fixes the detail.
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