What Most People Miss About Outlook Disk Space

Your inbox just hit 42,817 items. You’ve archived six years of emails, kept every receipt from vendor calls, and saved every calendar invite—even the ones you declined. Then your laptop starts wheezing during startup. You run Disk Cleanup. It finds 8.2 GB under 'Outlook Data Files'. You stare at the number. Where did that come from? And more importantly—can you trust it?

Quick Answer

Yes—Outlook takes up disk space, primarily through local PST/OST files and cached attachments. To see exactly how much: go to File > Account Settings > Account Settings… > Data Files tab, select each file, click Settings, then Compact Now. That opens a dialog showing current size and available compression savings.

Step-by-Step Walkthrough

Locate your data files: Click File > Account Settings > Account Settings…. Switch to the Data Files tab. You’ll see one or more entries like Outlook.pst (for POP/IMAP) or Outlook.ost (for Exchange/365). Note the Path column — this is where Outlook stores its local copy.
Check actual file size on disk: Right-click any path (e.g., C:\Users\Alex\AppData\Local\Microsoft\Outlook\Outlook.ost), choose Properties. Look at Size on disk, not just Size. OST files often inflate due to cached calendar blobs and embedded images — sometimes bloating 2–3× their logical size.
Compact the file: Back in the Data Files tab, select an entry, click Settings, then Compact Now. Outlook will pause syncing, shrink whitespace, and reclaim fragmented space. On large OSTs (>4 GB), this can take 2–12 minutes — don’t close Outlook mid-process.
Clear attachment cache: Go to File > Options > Advanced > Outlook Data File Settings. Click Attachment Settings. Uncheck “Save copies of attachments to the following folder” if enabled — this folder (often Attachments inside your Documents folder) can silently hoard hundreds of MB without warning.
Disable cached Exchange mode (if appropriate): For large mailboxes with limited local storage, consider turning off caching. Go to File > Account Settings > Account Settings… > Email tab > double-click your account > More Settings > Advanced tab. Uncheck “Use Cached Exchange Mode”. This eliminates the OST entirely — but requires constant network access and disables offline search.

Common Pitfalls

Assuming ‘Compact Now’ deletes emails: It doesn’t. It only removes empty blocks left behind after deletions — like defragging a hard drive. Your messages stay intact.
Running Compact while Outlook is syncing: Outlook may hang or cancel the operation. Wait until the status bar says “All folders are up to date” before clicking Compact Now.
Ignoring the Offline Synchronization Settings: In Account Settings > Change > More Settings > Advanced, people leave “Download shared folders” and “Download public folder favorites” checked — both pull down extra gigabytes of rarely-used content.
Using AutoArchive to “clean up” instead of managing size: AutoArchive moves old items to archive.pst — but that file still lives on your drive. It just swaps one big file for two.

Pro Tips

The beauty of this approach is that most disk bloat isn’t from email text — it’s from embedded images, meeting attachments, and cached calendar resources. Here’s what power users do:

Ctrl+Shift+G opens Go To Folder — type \Calendar and press Enter. Right-click the Calendar folder > Properties > Storage. You’ll see how many MB your calendar alone consumes. A single all-day recurring event with 50 attendees and a 5MB PowerPoint attachment? That’s stored 50 times — once per attendee instance.
Hold Shift while clicking ‘Empty Deleted Items’: This bypasses the recycle bin entirely and purges permanently — freeing space instantly. Works in both desktop and Outlook Web (but not mobile).
Turn off “Show pictures in messages” globally: Go to File > Options > Trust Center > Trust Center Settings > Automatic Download. Uncheck both boxes. This stops Outlook from downloading remote images (like marketing banners) — saving ~20–120 KB per message, across thousands of emails.

Troubleshooting

If Compact Now is grayed out or fails:

Outlook 2016 and earlier: Compaction only works on PST files — not OSTs. You’ll need to recreate the OST (remove/re-add account) to force a clean rebuild.
Outlook 365 (monthly channel): Compaction works on both PST and OST — but only if the file isn’t locked. Close OneDrive sync client first if your OST lives in a synced folder.
Admin-restricted environments: Some enterprise policies disable compaction or enforce mandatory caching. Run Outlook.exe /safe to test — if compaction works in Safe Mode, a third-party add-in is blocking it.
Outlook Web (OWA): No local disk impact — everything runs server-side. But cached browser data (images, scripts) can pile up. Clear site data manually: Settings > View all Outlook settings > General > Privacy > Clear browsing data.
iOS/Android Outlook apps: They store only recent messages locally (~30 days by default) and compress attachments aggressively. No manual compaction — but disabling “Download full attachments” in app settings cuts local footprint by 60–80%.

Feature	Outlook 365	Outlook 2019	Outlook 2016	Outlook Web
Compact PST/OST	✓ Both	✓ PST only	✓ PST only	✗ Not applicable
Attachment cache folder control	✓ Full path override	✓ Default only	✓ Default only	✗ N/A
Cached Exchange Mode toggle	✓ Available	✓ Available	✓ Available	✗ Server-only
AutoArchive settings location	File > Options > Advanced	File > Options > Advanced	Tools > Options > Other > AutoArchive	✗ Not supported
Keyboard shortcut for Go To Folder	Ctrl+Shift+G	Ctrl+Shift+G	Ctrl+Shift+G	Ctrl+G (search only)