How aspect ratios work
An aspect ratio is the relationship between the width and height of a rectangle, written as two numbers such as 16:9. It says nothing about actual size: a 1280 × 720 video and a 3840 × 2160 one are both 16:9. Keeping the ratio fixed is what stops images and video from looking stretched or squashed when they are resized.
How to simplify a ratio
Divide both numbers by their greatest common divisor. For 1920 × 1080 the greatest common divisor is 120, so the ratio is 1920 ÷ 120 : 1080 ÷ 120, or 16:9. Some sizes don't reduce neatly. 1366 × 768, a common laptop resolution, simplifies to 683:384, which is within a fraction of a percent of 16:9, so the calculator also tells you the nearest common ratio.
Finding a missing dimension
Pick a preset or type a ratio and it locks, so typing either side works out the other. You can also toggle the lock yourself with the padlock button between the two fields. The formula is simple:
height = width × (ratio height ÷ ratio width)For a 1200px wide 16:9 image, the height is 1200 × 9 ÷ 16 = 675px. With the ratio unlocked, typing both sides shows the ratio they make instead.
Common aspect ratios
| Ratio | Where it's used |
|---|---|
| 16:9 | HD and 4K video, YouTube, most monitors and TVs |
| 9:16 | Vertical video: Stories, Reels, TikTok, Shorts |
| 4:3 | Older monitors and TV, iPad screens, many compact cameras |
| 3:2 | 35mm film and most DSLR and mirrorless sensors |
| 1:1 | Square social posts, profile pictures, album art |
| 4:5 | Portrait posts on Instagram (1080 × 1350) |
| 16:10 | Many laptop screens |
| 21:9 | Ultrawide monitors and cinematic video |
"21:9" is a marketing name. Ultrawide monitors are usually 2560 × 1080 (64:27, about 2.37:1) or 3440 × 1440 (43:18, about 2.39:1), both a little wider than a true 21:9.
