---
title: "Compressed memory on a Mac · MacMelon"
description: "macOS compresses memory that has not been used recently before it resorts to swap. It is cheap, but not free. Sentinely counts it in each app's footprint."
url: "https://macmelon.com/glossary/compressed-memory"
---

# Compressed memory

Memory macOS has squeezed in place to make room, before it writes anything to disk.

Before macOS writes memory to swap, it compresses memory that has not been used recently and keeps it in RAM. Activity Monitor shows the total as Compressed. Compression costs some processor time each time the data is needed again, but far less than reading it back from disk.

Compressed memory still belongs to the app that owns it. That is why [Sentinely](https://macmelon.com/apps/sentinely) estimates with each process's [memory footprint](https://macmelon.com/glossary/memory-footprint), which includes its compressed memory, instead of resident size, which leaves it out.

## Apps

[No more Force Quit in the middle of a build. ### Sentinely](https://macmelon.com/apps/sentinely)

## Related guides

[### How to find what's slowing down your Mac](https://macmelon.com/guides/how-to-find-what-is-slowing-down-your-mac)

## Related use cases

[### Find out why your Mac is slow](https://macmelon.com/use-cases/find-out-why-your-mac-is-slow)

## Related terms

[How hard macOS is working to make room in memory. The signal that a Mac is actually short on memory, not just using it. ### Memory pressure](https://macmelon.com/glossary/memory-pressure)[Memory macOS has moved from RAM to the startup disk because RAM ran short. ### Swap](https://macmelon.com/glossary/swap-memory)[The memory macOS charges to a process, including memory it has compressed. Sentinely's unit for estimates and receipts. ### Memory footprint](https://macmelon.com/glossary/memory-footprint)