What aspect ratio means
The aspect ratio is the shape of a picture or screen: its width compared with its height, written as width:height. It says nothing about the size, so a 1280 × 720 video and a 3840 × 2160 one are both 16:9.
Working it out from width and height
Divide both numbers by their greatest common divisor, the largest number that divides both exactly.
- 1920 × 1080: both divide by 120, giving 16:9.
- 1024 × 768: both divide by 256, giving 4:3.
- 2400 × 1080, a common phone screen: both divide by 120, giving 20:9.
Some ratios come out as awkward numbers. 2560 × 1080 is exactly 64:27, which is sold as 21:9 "ultrawide". Dividing width by height gives a decimal you can compare directly: 16:9 is 1.78, 4:3 is 1.33.
Resizing without stretching
To keep the shape when you change the width, scale the height by the same factor:
new height = new width × original height ÷ original width
A 1920 × 1080 image resized to 1280 wide becomes 1280 × 1080 ÷ 1920 = 720 high. Changing only one side, or typing both sides freely, stretches or squashes the picture.
Common aspect ratios
- 16:9: TVs, monitors, YouTube and most video.
- 4:3: older screens, many tablets and compact cameras.
- 3:2: most DSLR and mirrorless camera photos, and 6 × 4 in prints.
- 1:1: square social media posts.
- 9:16: vertical video for phones, Stories, Reels and TikTok.
- 4:5: portrait posts on Instagram.
Changing the ratio
An image can only change ratio by cropping part of it away or by adding bars or a background around it. Cropping keeps the picture sharp but loses its edges; adding bars keeps everything but leaves empty space.
Tools
The Aspect Ratio Calculator reduces any width and height to a ratio and gives the missing side for a new size. Resize Image keeps the ratio for you, and the PPI Calculator works out a screen's ratio and pixel density together.