---
title: "Swap memory on a Mac · MacMelon"
description: "Swap is memory macOS has written to disk because RAM is full. A large swap file is not a problem on its own; reading from it often is. Sentinely measures both."
url: "https://macmelon.com/glossary/swap-memory"
---

# Swap

Memory macOS has moved from RAM to the startup disk because RAM ran short.

When apps need more memory than the Mac has, and compression is not enough, macOS writes some memory out to files on the startup disk. That is swap. Activity Monitor shows the total as Swap Used. The SSD is fast, but much slower than RAM, so an app whose memory was swapped out pauses while it comes back.

Swap used on its own is context, not a verdict. Memory that was swapped out and never needed again costs little. What you feel is the swap-in rate: how often the Mac reads memory back from disk. [Sentinely](https://macmelon.com/apps/sentinely) reports both, and its [measured receipts](https://macmelon.com/glossary/measured-receipt) compare them before and after a fix. Swap also takes disk space, which is one place memory and [Disk Housekeeper](https://macmelon.com/apps/disk-housekeeper) territory meet.

## 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)[### When Slack, Teams, or Discord slows your Mac](https://macmelon.com/use-cases/slack-teams-and-discord-slowing-your-mac)

## 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 squeezed in place to make room, before it writes anything to disk. ### Compressed memory](https://macmelon.com/glossary/compressed-memory)[Sentinely's record of a fix: what was freed and what changed for you, measured after the fix rather than promised before it. ### Measured receipt](https://macmelon.com/glossary/measured-receipt)