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

# What is compression ratio?

By Nikhil Barik · Published November 3, 2026

## Quick answer

Compression ratio compares a file's original size with its compressed size. A 100 MB file reduced to 25 MB has a ratio of 4 to 1.

## What it is

You divide the original size by the new size. Higher means more shrinking. Lossy formats reach higher ratios than lossless ones.

## Why it matters on a Mac

Files that are already compressed, such as MP4 videos, JPEGs and many PDFs, have little room left, so the ratio can be small.

## How to check it on macOS

Select the original and the compressed copy in Finder and press Command-I on each. Compare the sizes.

## How Toolbox handles it

Compress Video, Compress Images and Compress PDF write smaller copies to a new folder and keep your originals. If a PDF cannot be made smaller, Toolbox keeps the original as it is.

## Frequently asked questions

**What is a good compression ratio?**

It depends on the content. Photos and video can shrink a lot, plain text PDFs very little.

**Does higher compression mean lower quality?**

For lossy formats, usually yes.

**Why did my PDF not get smaller?**

It was mostly text or already compressed.

## Tools mentioned

- [**Compress Video**: Shrink video files while keeping them looking great.](https://mactoolbox.app/compress-video-mac)
- [**Compress Images**: Make photos smaller without visible loss in quality.](https://mactoolbox.app/compress-images-mac)
- [**Compress PDF**: Make PDFs smaller by recompressing their images.](https://mactoolbox.app/compress-pdf-mac)

## Keep reading

- [Compress Video](https://mactoolbox.app/compress-video-mac)
- [Compress Images](https://mactoolbox.app/compress-images-mac)
- [Compress PDF](https://mactoolbox.app/compress-pdf-mac)
- [Lossless vs lossy](https://mactoolbox.app/glossary/lossless-vs-lossy)