PX to REM converter
Pixels to rem and back at any root font size, with a table of the common sizes ready to copy.
Aspect ratio calculator
The ratio of any width and height, or the missing side for 16:9, 4:3, 9:16 and the rest.
- Choose what to work out
- Enter a width and height, or a ratio and one side
- Read the ratio, or the missing side
Example figures. Put in yours.
Aspect ratio Height Width
- As a decimal
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- Shape
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- Also called
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- Nearest standard
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The calculator needs JavaScript. The formula is below if you would rather work it out by hand.
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This aspect ratio calculator gives you the ratio of any width and height, simplified. It names the ratio if it has a name. If it doesn’t, it shows the nearest standard.
Need the missing side instead? Pick 16:9, 4:3, 9:16 or your own ratio, enter one side, and it gives you the other.
How to calculate aspect ratio (the aspect ratio formula)
Divide both sides by the largest number that goes into both. That’s their greatest common divisor.
For 1920 × 1080 that number is 120. 1920 ÷ 120 is 16 and 1080 ÷ 120 is 9. So the ratio is 16:9.
Not every size is that tidy. For 1366 × 768 the divisor is only 2, which gives 683:384. That’s 1.779 to 1. Close enough to 16:9 (1.778 to 1) that it’s sold as widescreen.
Finding the missing side for 16:9 and other ratios
| To find | Formula | Example at 16:9 |
|---|---|---|
| Height | Width × second number ÷ first number | 1280 × 9 ÷ 16 = 720 |
| Width | Height × first number ÷ second number | 1080 × 16 ÷ 9 = 1920 |
Common aspect ratios
| Ratio | As a decimal | Where you’ll see it |
|---|---|---|
| 16:9 | 1.78:1 | Widescreen video, HD and 4K screens, YouTube |
| 9:16 | 0.56:1 | Vertical video: Stories, Reels, TikTok, Shorts |
| 4:3 | 1.33:1 | Older screens and TV, many tablets |
| 3:2 | 1.5:1 | Photography, many cameras |
| 1:1 | 1:1 | Square images and posts |
| 4:5 | 0.8:1 | Portrait images and posts |
| 16:10 | 1.6:1 | Laptops and monitors |
| 21:9 | 2.33:1 | Ultrawide monitors and cinema |
Making an image for social media? Each platform asks for its own size. Our social media image sizes tool lists them all and crops your picture to the one you need.
Does the unit matter?
No. A ratio compares the two sides, so the unit cancels out. Pixels, centimetres and inches all give the same answer, as long as both sides use the same one.
A print 20cm × 15cm and an image 800 × 600 pixels are both 4:3.
Aspect ratio on websites
On a website, the shape matters twice. Images cropped to the wrong shape look wrong. And images with no set shape make the page jump as they load. Google measures that jump as layout shift.
The fix is simple. Give every image its width and height, or set the shape in CSS with the aspect-ratio property, for example aspect-ratio: 16 / 9. The browser then keeps the space free before the image arrives.
It’s one of the details we build into every site. Our WordPress image guide covers the rest.
A website is judged by what it converts, not by what it wins.
Client results, measured in the numbers the business cares about.
See the workQuestions people ask.
How do you calculate aspect ratio from width and height?
Divide both by their greatest common divisor. For 1920 × 1080 that is 120, and 1920 ÷ 120 by 1080 ÷ 120 gives 16:9. The calculator does it for any size.
How do I work out the height for a 16:9 image?
Multiply the width by 9 and divide by 16. A 1280px-wide image at 16:9 is 720px high. For any ratio, height = width × second number ÷ first number.
What aspect ratio is 1920 × 1080?
16:9, the standard widescreen ratio. 1280 × 720, 2560 × 1440 and 3840 × 2160 are 16:9 too.
What aspect ratio is 1366 × 768?
Exactly 683:384, which is 1.779 to 1. That is close enough to 16:9 (1.778 to 1) that it is sold and treated as 16:9.
Does it matter if I use pixels, centimetres or inches?
No. A ratio compares the two sides, so the unit cancels out. Use whatever you measured in, as long as both sides are in the same unit.
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