How to Shrink and Compact a Large Outlook OST File

What Causes This?

An Outlook OST file grows as Outlook caches more mailbox data for offline use, including email, attachments, calendars, and shared folders. Over time, deleted items and mailbox changes do not always reduce the file size right away, so the OST can stay much larger than it needs to be. Large mailboxes, long sync history, and cached shared mailboxes are the most common reasons it becomes oversized.

Common Symptoms

• Outlook feels slow when opening folders, searching, or syncing
• Your C: drive keeps losing free space for no obvious reason
• Send/receive takes longer than usual or stalls
• Outlook shows sync errors or freezes during startup
• The OST file in your user profile is several GB larger than expected

How to Fix It Manually

1. Check the size of your OST file

   1. Close Outlook completely.
   2. Open File Explorer with Win + E.
   3. Go to: C:\Users\YourUserName\AppData\Local\Microsoft\Outlook
   4. If you do not see AppData, click View > Show > Hidden items.
   5. Right-click your .ost file, choose Properties, and note the size.

2. Empty deleted and junk mail folders in Outlook

   1. Open Outlook.
   2. Right-click Deleted Items and choose Empty Folder.
   3. Right-click Junk Email and choose Empty Folder.
   4. Remove large emails with attachments from other folders if you no longer need them.
   5. Close Outlook after cleanup.

3. Compact the OST file

   1. Reopen Outlook.
   2. Click File > Account Settings > Account Settings.
   3. Open the Data Files tab.
   4. Select your Outlook data file, then click Settings.
   5. Click Advanced > Outlook Data File Settings.
   6. Click Compact Now.
   7. Wait for the process to finish. This can take several minutes if the file is large.

4. Reduce how much mail Outlook keeps offline

   1. In Outlook, go to File > Account Settings > Account Settings.
   2. On the Email tab, select your account and click Change.
   3. Find the Mail to keep offline slider.
   4. Move it from All to a shorter period such as 12 months or 6 months.
   5. Click Next, then Done, and restart Outlook.

5. Turn off shared folder caching if it is not needed

   1. Go to File > Account Settings > Account Settings.
   2. Select your account and click Change > More Settings.
   3. Open the Advanced tab.
   4. Uncheck Download shared folders if you do not need shared mail cached offline.
   5. Click OK, then restart Outlook.

6. Rebuild the OST file if it stays unusually large

   1. Close Outlook.
   2. Press Ctrl + Shift + Esc to open Task Manager and make sure no Outlook process is still running.
   3. Go back to C:\Users\YourUserName\AppData\Local\Microsoft\Outlook.
   4. Rename the OST file to something like mail-old.ost.
   5. Open Outlook again. It will create a new OST file and re-sync your mailbox.
   6. After confirming Outlook works normally, delete the old renamed file to free space.

Fix It Automatically with Kudu

Kudu can quickly spot oversized Outlook OST files, show you where the disk space is going, and help you clean up the underlying storage problem without digging through hidden folders. It is an easier way to catch bloated mail caches and other large files that slow down your PC.

Download Kudu Free →