What Is an Aspect Ratio?

An aspect ratio expresses the proportional relationship between width and height as two numbers separated by a colon. A ratio of 16:9 means the image is 16 units wide for every 9 units tall. Aspect ratios are fundamental in video production, photography, web design, and display hardware.

Common Aspect Ratios

16:9 is the standard for HD and 4K video, computer monitors, and most modern TVs. 4:3 was the standard for analogue TV and many older monitors. 21:9 is used for ultrawide displays and cinematic video. 1:1 (square) is popular on Instagram and other social platforms. 9:16 is the vertical portrait format used for Reels, TikTok, and Stories.

Finding a Missing Dimension

Cross-multiplication makes it straightforward. If you know an image is 16:9 and 1920 px wide, the height is (1920 ÷ 16) × 9 = 1080 px. The tool does this calculation instantly in your browser.

Simplifying a Ratio

Any W×H pair can be simplified by dividing both numbers by their greatest common divisor (GCD). 1920 and 1080 share a GCD of 120, giving 16:9. This is useful when checking whether a crop or thumbnail matches the source aspect ratio.

Photography and print aspect ratios

Ratio	Common use	Example resolution
3:2	Full-frame DSLR / mirrorless cameras	6000 × 4000 px
4:3	Micro Four Thirds cameras; older monitors	4032 × 3024 px
1:1	Square medium-format film; Instagram grid posts	3000 × 3000 px
16:9	HD and 4K video; widescreen monitors	1920 × 1080 px
9:16	Mobile-first vertical video (Reels, TikTok, Stories)	1080 × 1920 px
21:9	Ultrawide monitors; anamorphic cinema	3440 × 1440 px
5:4	Large-format film (4×5 inch); older CRT monitors	1280 × 1024 px
4:5	Portrait posts on Instagram (optimal for feed)	1080 × 1350 px