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Aspect ratio is the proportional relationship between a width and a height — 16:9 for most videos, 4:3 for old TVs, 1:1 for a square post. Knowing the ratio of a set of dimensions matters whenever you resize an image, choose a screen, or design to a standard format without distorting it.
Enter a width and a height, and the simplified aspect ratio plus the decimal ratio appear at once.
How is it calculated?
How it's found
The aspect ratio is the width and height reduced to their simplest whole-number form by dividing both by their greatest common divisor (GCD). 1920 and 1080 share a GCD of 120, so 1920:1080 simplifies to 16:9.
Common aspect ratios
| Ratio | Used for |
|---|---|
| 16:9 | HD/4K video, most modern screens |
| 4:3 | old TVs and monitors, some cameras |
| 1:1 | square social posts (Instagram) |
| 21:9 | ultrawide monitors, cinema |
| 3:2 | classic photography, many prints |
| 9:16 | vertical video (Stories, Reels, TikTok) |
Why it matters
To resize without distortion, the new dimensions must keep the same ratio. If an image is 1600×900 (16:9) and you set the width to 800, the height must be 450 to stay 16:9 — otherwise it stretches. The ratio (and its decimal form, 1.778 for 16:9) is what you hold constant.
Where it helps
- Resizing images or video without stretching
- Choosing a screen or export format
- Designing to a platform's required ratio (thumbnails, ads, stories)
Worked example
A Full HD screen is 1920×1080. The greatest common divisor of 1920 and 1080 is 120, so dividing both gives 1920÷120 : 1080÷120 = 16:9 — the standard widescreen ratio, with a decimal value of 1.778. Compare an older 1280×1024 monitor: its GCD is 256, giving 5:4 (decimal 1.25), which is why 4:3 content looked slightly stretched on those screens. Knowing the ratio lets you resize safely: to shrink the 1920×1080 image to 1280 wide, keep 16:9 and the height becomes 1280 ÷ (16/9) = 720.
FAQ
How do I calculate an aspect ratio?+
Divide the width and height by their greatest common divisor to reduce them to simplest form. 1920×1080 has a GCD of 120, giving 16:9. The tool does this instantly.
What is 1920×1080 as an aspect ratio?+
16:9, the standard widescreen ratio (decimal 1.778). It's used for HD and 4K video and most modern displays.
How do I resize an image without distorting it?+
Keep the same aspect ratio. If the original is 16:9 and you set a new width, multiply by 9/16 for the height (or divide the width by 1.778). Changing only one dimension stretches the image.
What is the aspect ratio of a vertical video?+
9:16 — the portrait version of 16:9, used for Stories, Reels and TikTok. A 1080×1920 frame is 9:16.
What does the decimal ratio mean?+
It's the width divided by the height: 16:9 is 1.778, 4:3 is 1.333, 1:1 is 1.0. The decimal is handy for quick resize maths and comparing formats.
Why did 4:3 content look stretched on old monitors?+
Many old monitors were 5:4 (1280×1024), slightly taller than 4:3 (1.25 vs 1.333). Displaying 4:3 content full-screen stretched it a little to fit.