# Aspect Ratio Calculator

> An aspect ratio calculator finds the missing dimension when you know one side and the ratio you need. Enter a width at 16:9 and it returns the matching height, and entering any two dimensions gives you their simplified ratio.

Source: https://rankcert.com/tools/aspect-ratio-calculator
Free, no signup. Updated: 2026-09-26

## How to use it

1. Pick a common ratio, or type any width and height.
2. Change either dimension and the other follows the locked ratio.
3. Read the simplified ratio and megapixel count underneath.

## Common aspect ratios and sizes

A ratio is a shape, not a size. Every resolution listed against a ratio has the same shape and different sharpness.

| Ratio | Decimal | Common sizes | Used for |
| --- | --- | --- | --- |
| 16:9 | 1.778 | 1280x720, 1920x1080, 3840x2160 | Video, YouTube, most displays |
| 1.91:1 | 1.910 | 1200x630 | Open Graph and social preview cards |
| 4:3 | 1.333 | 1024x768, 1600x1200 | Older displays, some slides |
| 1:1 | 1.000 | 1080x1080 | Instagram posts, avatars |
| 4:5 | 0.800 | 1080x1350 | Instagram portrait posts |
| 9:16 | 0.563 | 1080x1920 | Reels, Stories, TikTok |
| 3:2 | 1.500 | 6000x4000 | Most DSLR and mirrorless cameras |
| 21:9 | 2.370 | 2560x1080, 3440x1440 | Ultrawide monitors |

## Questions

### What is 16:9 in pixels?

Any pair where width divided by height is 1.778 - 1280x720, 1920x1080, 2560x1440 and 3840x2160 are all 16:9. The ratio does not specify a size.

### How do I calculate an aspect ratio from two dimensions?

Divide both by their greatest common divisor. 1920 and 1080 share a divisor of 120, giving 16:9.

### What aspect ratio should a YouTube thumbnail be?

16:9, at 1280x720. Instagram posts are 1:1 or 4:5, Stories and Reels are 9:16, and an Open Graph image is 1.91:1 at 1200x630.

### What is the difference between aspect ratio and resolution?

Ratio is the shape; resolution is the number of pixels. 1280x720 and 3840x2160 have identical shapes and very different sharpness.

### How do I keep an aspect ratio in CSS?

Use the aspect-ratio property: aspect-ratio: 16 / 9. It is supported everywhere now and replaces the old padding-top percentage hack.

### What is 21:9 actually?

A marketing name for ultrawide monitors, which are usually 2560x1080 (64:27) or 3440x1440 (43:18). Neither is exactly 21:9.

### Why does my video have black bars?

The video's ratio does not match the display's, so the player letterboxes it rather than cropping or stretching. Matching the ratio at export removes them.

### How many megapixels is 4K?

About 8.3 for 3840x2160. 1080p is 2.1 and 8K is 33.2.

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