# Aspect ratio calculator

> Find the aspect ratio of any width and height, or the missing side for a ratio like 16:9 or 4:3. Common sizes included. Free, runs in your browser.

Canonical: https://toolspace.tech/aspect-ratio-calculator/
Last updated: 2026-10-25

Enter a width and height to get the simplified ratio, or pick a ratio like 16:9 and one side to get the other. Common sizes for the ratio are listed below.

This is an interactive tool that runs in the browser: [open the Aspect ratio calculator](https://toolspace.tech/aspect-ratio-calculator/) to use it. Nothing typed into it is uploaded.

## Worked example

Simplify a monitor size, then size a video for the same ratio.

**input**

```
Width 2560, height 1440
Ratio 16:9, width 1280
```

**result**

```
2560 × 1440 → 16:9 (1.7778)
16:9 at 1280 wide → 1280 × 720
```

## Common mistakes

### Rounding the ratio, not the size

Work out the missing side from the full ratio and round only the result, or small errors add up.

### Odd pixel counts for video

Many video encoders need even widths and heights. If the result is odd, step to the nearest even size.

### Mixing up width and height

9:16 is portrait and 16:9 is landscape. The first number is always the width.

### Stretching instead of cropping

Changing an image’s ratio means cropping or adding bars. Resizing to a new ratio without either distorts it.

## How it works

1. **Enter a size:** Type the width and height in pixels (or any unit). The ratio is reduced to whole numbers, with its decimal value.
2. **Find a missing side:** Pick a preset or type a ratio such as 1.85:1, then enter a width or a height to get the other.
3. **Check common sizes:** The table lists familiar screen and media sizes at that ratio, and marks those that are only close to it.

## Questions

### How do I calculate an aspect ratio?

Divide the width and height by their greatest common divisor. For 1920 × 1080 that is 120, which gives 16:9.

### Is 21:9 really 21:9?

Not exactly. “21:9” monitors are usually 2560 × 1080 (64:27) or 3440 × 1440 (43:18), both a little wider than 21:9. The table marks such sizes as close rather than exact.

### Why is 1366 × 768 not exactly 16:9?

It reduces to 683:384, about 1.779. It is sold as 16:9 (1.778) because it is within a fraction of a pixel of it.

### What ratio should I use for social video?

9:16 (such as 1080 × 1920) fills a phone screen held upright; 1:1 and 4:5 (1080 × 1350) suit feeds; 16:9 suits landscape players.

## Related

- [Image resizer](https://toolspace.tech/image-resizer/)
- [Image compressor](https://toolspace.tech/image-compressor/)
- [Favicon generator](https://toolspace.tech/favicon-generator/)