Source: https://mactoolbox.app/glossary/aspect-ratio

# What is aspect ratio?

By Nikhil Barik · Published November 3, 2026

## Quick answer

Aspect ratio is the relationship between an image or video's width and height, written like 16:9. Common ratios are 16:9 for widescreen video, 4:3 for older screens and 1:1 for square.

## What it is

A 1920 by 1080 video is 16:9. Divide width by height to get the ratio, then simplify. A vertical video for phones is often 9:16.

## Why it matters on a Mac

Changing only the width or height stretches the picture. If the ratio differs from the target, you must add bars, crop or accept distortion.

## How to check it on macOS

In Preview, Tools > Adjust Size shows width and height. In QuickTime Player, the Movie Inspector shows the frame size.

## How Toolbox handles it

Resize Video has presets such as 1080p, square and vertical, and with Keep aspect ratio on you can Fit within size, Add black bars or Crop to fill. Resize Images has a Keep proportions option.

## Frequently asked questions

**What is 16:9 in pixels?**

1920 by 1080 and 1280 by 720 are both 16:9.

**How do I fit a wide video into a vertical frame?**

Crop to fill, or add black bars to keep the whole picture.

**Why did my custom video size change slightly?**

Encoders need even dimensions, so odd sizes are rounded down.

## Tools mentioned

- [**Resize Video**: Change the dimensions of videos to an exact size.](https://mactoolbox.app/resize-video-mac)
- [**Resize Images**: Scale photos by percentage, longest side or exact size.](https://mactoolbox.app/resize-images-mac)
- [**Crop & Rotate Video**: Crop, rotate and flip a video.](https://mactoolbox.app/crop-video-mac)

## Keep reading

- [Resize Video](https://mactoolbox.app/resize-video-mac)
- [Resize Images](https://mactoolbox.app/resize-images-mac)
- [Crop & Rotate Video](https://mactoolbox.app/crop-video-mac)