Aspect ratio formulas
Ratio = (width ÷ GCD) : (height ÷ GCD)
New height = new width × original height ÷ original width
New width = new height × original width ÷ original height
Worked example
A 1920 × 1080 video has a greatest common divisor of 120, so its ratio is 16:9. To shrink it to 1280 px wide, the new height is 1280 × 1080 ÷ 1920 = 720 px.
Common aspect ratios
| Ratio | Common sizes | Used for |
|---|
| 16:9 | 1280×720, 1920×1080, 3840×2160 | HD TV, YouTube, most monitors |
| 9:16 | 1080×1920 | Stories, Reels, TikTok, Shorts |
| 4:3 | 1024×768, 1600×1200 | Older TVs, iPads, many photos |
| 3:2 | 6000×4000 | DSLR and mirrorless photos |
| 1:1 | 1080×1080 | Square social posts |
| 4:5 | 1080×1350 | Instagram portrait posts |
| 16:10 | 1920×1200, 2560×1600 | Laptops and some monitors |
| 21:9 | 2560×1080, 3440×1440 | Ultrawide monitors, cinema |
Simplifying other ratios? Use the ratio calculator. Making pixel art? Try the pixel circle generator.
Last reviewed October 2026.
Frequently asked questions
What is an aspect ratio?
An aspect ratio is the relationship between width and height, written as width:height. A 1920 × 1080 screen is 16:9, meaning it is 16 units wide for every 9 units tall. The ratio stays the same however big or small the image is.
How do I calculate an aspect ratio?
Divide the width and height by their greatest common divisor. For 1920 × 1080 the greatest common divisor is 120, so the ratio is 1920 ÷ 120 : 1080 ÷ 120 = 16:9.
How do I resize an image without stretching it?
Keep the same ratio. Enter the original size, then type the new width and the calculator gives the height (or the other way round). The formula is new height = new width × original height ÷ original width.
Why is 2560 × 1080 called 21:9?
"21:9" is a marketing name for ultrawide screens. The exact ratio of 2560 × 1080 is 64:27 (about 2.37:1), which is close to 21:9 (2.33:1). The calculator shows both the exact ratio and the nearest common name.
What aspect ratio should I use for social media?
Common choices are 16:9 for YouTube and landscape video, 9:16 for Stories, Reels, TikTok and Shorts, 1:1 for square posts and 4:5 for portrait feed posts on Instagram.