When Outlook takes longer to open, freezes during search, or lags every time new mail arrives, an oversized OST file is often the reason. As your mailbox grows, the offline copy on your computer grows with it, and past a certain point that file starts to drag Outlook down. The good news is that you can reduce the OST file size and get Outlook running smoothly again without deleting anything you need. This guide shows you how, step by step.
Table Of Contents
- 1 Quick answer
- 2 Why does a large OST file slow down Outlook?
- 3 How large is too large?
- 4 Signs your OST file is too big
- 5 Before you start
- 6 How to reduce OST file size and speed up Outlook
- 6.1 Method 1: Shorten the Cached Exchange Mode sync window
- 6.2 Method 2: Empty the Deleted Items, Junk, and Sync Issues folders
- 6.3 Method 3: Archive or convert old mail to a PST file
- 6.4 Method 4: Remove or save large attachments
- 6.5 Method 5: Compact the OST file to reclaim space
- 6.6 Method 6: Rebuild the OST after cleanup
- 7 When a large OST becomes a risk: recover or convert it
- 8 How to keep your OST file small
- 9 Conclusion
- 10 Frequently Asked Questions
Quick answer
To reduce your OST file size and speed up Outlook, shorten the Cached Exchange Mode sync window so Outlook keeps fewer months of mail offline, empty the Deleted Items, Junk, and Sync Issues folders, remove or save large attachments, and then compact the OST file to reclaim the freed space. For mailboxes with years of old mail, archive or convert the older items to a PST file so they leave the OST entirely. These steps shrink the file and restore performance while keeping every email safe.
Why does a large OST file slow down Outlook?
An OST file (Offline Storage Table) is the local copy of your mailbox that Outlook builds when your account runs in Cached Exchange Mode with Exchange, Microsoft 365, or Outlook.com. Every email, attachment, contact, and calendar item you keep offline lives inside this single file.
Outlook has to read, write, index, and search that one file constantly. When it swells into several gigabytes, each of those operations takes longer, which shows up as slow startups, laggy search, and delays during send and receive. Older computers and drives with limited free space feel it soonest.
If you want a refresher on how the offline file differs from a portable PST file, see our guide on the difference between OST and PST files.
How large is too large?
Outlook can technically handle large OST files, with a default ceiling of around 50 GB. Performance, though, usually starts to suffer long before that, often once the file grows into the range of several gigabytes. There is no single magic number, because it depends on your hardware, your free disk space, and how many items sit in each folder. A practical rule is simple: if Outlook feels slow and your OST file is large, reducing its size will help.
Signs your OST file is too big
- Outlook is slow to open or hangs on the loading screen.
- Search results appear slowly or come back incomplete.
- Send and receive stalls or freezes Outlook.
- Your computer warns about low disk space on the drive holding the OST file.
- Outlook shows errors that point to the data file, such as the offline file being unreadable.
A very large OST file is also more likely to become corrupt. If yours has already started throwing errors, our guide on how to fix the file .ost cannot be opened error walks through the repair, and the wider list of common Outlook errors and their solutions covers the rest.
Before you start
It helps to know where the file lives and how big it currently is, so you can measure your progress. Our guide on how to find your OST file location in Outlook shows the exact path for every version. Note the current size before you begin, then check it again after each step.
How to reduce OST file size and speed up Outlook
Work through these methods in order. The first one alone often makes the biggest difference.
Method 1: Shorten the Cached Exchange Mode sync window
This is the single most effective lever. By telling Outlook to keep fewer months of mail offline, you cut the OST size without touching anything on the server.
- Go to File, then Account Settings, then Account Settings again.
- Double click your Exchange or Microsoft 365 account.
- Find the slider labelled “Mail to keep offline” and drag it to a shorter period, such as 3 months or 1 month.
- Click Next, then Done, and restart Outlook.
Anything outside that window stays safely on the server and is still searchable online. Only the local copy shrinks.
Method 2: Empty the Deleted Items, Junk, and Sync Issues folders
Deleted and junk mail keep taking up space in the OST until you clear them, and the Sync Issues folders quietly bloat the file over time.
- Right click the Deleted Items folder and choose Empty Folder.
- Do the same for the Junk Email folder.
- In the folder list, open the Sync Issues group and delete the contents of its subfolders.
Method 3: Archive or convert old mail to a PST file
Moving older emails out of the OST is the cleanest way to keep the file small while preserving everything. A PST file is a separate, portable data file you can store anywhere and open whenever you need those older messages.
You can use Outlook AutoArchive, or export a date range manually. For mailboxes that are already very large or partly inaccessible, converting the OST to PST with a dedicated tool is faster and safer, because it reads the file directly and lets you choose exactly what to move. Our step by step guide on how to convert OST to PST covers both the manual method and the automated one.
Method 4: Remove or save large attachments
A handful of large attachments can account for a surprising share of your OST size.
- Sort a folder by size, or search for large items, to find the biggest messages.
- Save the attachments you want to keep to a folder on your computer.
- Remove the attachment from the email, or delete the message if you no longer need it.
Method 5: Compact the OST file to reclaim space
Deleting items marks the space as free, but Outlook does not always shrink the file automatically. Compacting recovers that empty space.
- Go to File, then Account Settings, then Account Settings.
- On the Data Files tab, select your account and click Settings.
- Open the Advanced tab and click Outlook Data File Settings.
- Click Compact Now and wait for the process to finish.
Compacting works best after you have already emptied folders and removed attachments, so do it last.
Method 6: Rebuild the OST after cleanup
If the file is still large after compacting, you can force Outlook to build a fresh, lean copy. This works only when your mailbox is active on the server, because Outlook downloads a new OST from it.
- Close Outlook completely.
- Browse to the OST folder and rename the file from outlook.ost to outlook.old.
- Reopen Outlook. It creates a new OST using your current, shorter sync window.
When a large OST becomes a risk: recover or convert it
Very large OST files are more prone to corruption, and a corrupt or orphaned file will not compact or rebuild cleanly. If your OST has grown huge and Outlook can no longer open it, the safe move is to extract the mailbox into a healthy PST first.
An OST to PST Converter reads the offline file without needing Outlook or a live connection and exports everything to a fresh PST you can open anywhere. For files that are already damaged, an OST Recovery tool repairs the structure and restores your emails, contacts, calendars, and attachments before you save them. Both give you a clean copy of your data, which you can then keep offline in a smaller, manageable form.
How to keep your OST file small
A few habits stop the file from ballooning again.
- Keep the Cached Exchange Mode sync window short, and extend it only when you truly need older mail offline.
- Empty the Deleted Items and Junk folders on a regular schedule.
- Store large attachments outside Outlook rather than inside the mailbox.
- Archive or convert old mail to PST every few months as a routine cleanup and backup.
- Watch your free disk space so the drive holding the OST never runs tight.
Conclusion
A slow Outlook is usually a symptom, and an oversized OST file is often the cause. Start by shortening the sync window, clear out deleted, junk, and sync folders, strip out large attachments, and compact the file to reclaim the space. For mailboxes carrying years of history, move the old mail into a PST so the offline file stays lean. Keep up the simple maintenance habits and Outlook will stay fast, with all of your email intact.
Frequently Asked Questions
It grows with your mailbox. Every email, attachment, contact, and calendar item you keep offline is stored inside the OST file, so years of mail, big attachments, and a wide sync window all add up. Deleted and junk items also keep taking space until you clear them.
Modern Outlook uses a default limit of around 50 GB for the offline file. Performance usually declines well before that point, so it is best to keep the file much smaller for smooth day to day use.
Yes. Outlook reads, indexes, and searches the entire file, so as it grows those tasks take longer, which appears as slow startups, laggy search, and delays during send and receive.
Go to File, then Account Settings, then Account Settings. On the Data Files tab, select your account, click Settings, open the Advanced tab, click Outlook Data File Settings, then click Compact Now. Empty your folders and remove large attachments first so there is more space to reclaim.
No, not when you do it correctly. Shortening the sync window only removes the local copy of older mail, which stays safe on the server. Archiving or converting old mail moves it into a PST file, so it is preserved rather than deleted.
Archive them or convert them to a PST file. You can use Outlook AutoArchive, export a date range manually, or use an OST to PST Converter to move the older items into a separate, portable file. See our convert OST to PST guide for the steps.
Yes. Bigger files are more likely to develop errors, and a corrupt OST may refuse to open. If that happens, our guide on the file .ost cannot be opened error explains how to repair or recover it.
You can. Switching to Online Mode removes the OST file entirely, since Outlook then reads directly from the server. The trade off is that you need a constant connection and lose offline access, so most users prefer to keep Cached Exchange Mode with a shorter sync window instead.
- How to Reduce OST File Size When Outlook Runs Slow - August 4, 2026
- How to Fix “The File .ost Cannot Be Opened” Error in Outlook - August 3, 2026
- How to Clear Gmail Storage Without Deleting Anything - May 13, 2025