How To Check Mailbox Size In Outlook 365 : Outlook 365 Mailbox Size Limits

by

Your mailbox can quickly fill with thousands of messages, and Outlook 365 provides a simple way to see exactly how much space you have left. Learning how to check mailbox size in outlook 365 is essential for avoiding those annoying “mailbox full” warnings that stop you from sending or receiving emails. When your mailbox gets too big, Outlook might run slowly, and you could miss important messages. This guide walks you through every method, from the desktop app to the web version, so you always know your storage status.

Microsoft gives each Office 365 user a generous mailbox quota, but attachments, large emails, and deleted items that still sit in the Recoverable Items folder can eat up space fast. By checking your size regularly, you can clean up before hitting the limit. Let’s get into the step-by-step process.

Why Mailbox Size Matters In Outlook 365

Your mailbox storage isn’t infinite. Most Outlook 365 plans offer 50 GB to 100 GB of space per user, but once you cross the limit, you can’t send new messages. You might also struggle to receive emails. Knowing your current usage helps you plan ahead.

A full mailbox can also slow down search and sync across devices. If you use Outlook on your phone, tablet, and computer, a bloated mailbox means longer load times. Regular checks keep everything running smooth.

Another hidden issue: the Deleted Items folder and the Recoverable Items folder both count toward your quota. Even after you delete something, it might still take up space until you empty those folders. That’s why checking size is the first step to reclaiming storage.

How To Check Mailbox Size In Outlook 365

Now we get to the core of this guide. The exact phrase “How To Check Mailbox Size In Outlook 365” covers several methods, depending on which version of Outlook you use. Below, I break down each approach with clear steps.

Method 1: Using The Outlook Desktop App (Windows)

This is the most common method for people who use the full Outlook client. Follow these steps exactly:

  1. Open Outlook on your Windows computer.
  2. Click on the File tab in the top-left corner.
  3. In the Info section, click the Tools button (it looks like a small gear or wrench).
  4. From the dropdown menu, select Mailbox Cleanup.
  5. A new window opens. Click the View Mailbox Size button.
  6. Outlook will take a few seconds to calculate your size. You’ll see a breakdown by folder: Inbox, Sent Items, Deleted Items, and more.
  7. The total size appears at the top. It shows both the size and the percentage of your quota used.

If you don’t see the Tools button, try this alternative path: File > Account Settings > Account Settings > double-click your email account > click More Settings > go to the Advanced tab > click Mailbox Size. This works in older versions too.

One common mistake: people forget that the “Total size” includes all subfolders. So if you have a folder with 10,000 old newsletters, that counts. The breakdown helps you find the biggest space hogs.

Method 2: Using Outlook On The Web (OWA)

If you prefer a browser, or if you don’t have the desktop app installed, the web version works just as well. Here’s how to check mailbox size in Outlook 365 using OWA:

  1. Go to outlook.office.com and sign in with your work or school account.
  2. In the top-right corner, click the Settings gear icon (it looks like a small cog).
  3. A sidebar opens. At the bottom of that sidebar, click View all Outlook settings.
  4. In the Settings window, click on General in the left menu.
  5. Then click Storage.
  6. You’ll see a pie chart showing your used space and total quota. Below that, a list of folders shows how much each one uses.

This method is fast and doesn’t require any downloads. The pie chart updates in real time, so you can see the effect of deleting emails immediately. Note that the web version sometimes shows slightly different numbers than the desktop app due to caching, but the difference is usually small.

Method 3: Using The Outlook Mobile App

On your phone, checking mailbox size is less direct, but still possible. The mobile app doesn’t have a built-in “mailbox size” button, but you can use the web version through your phone’s browser. Alternatively, follow these steps for a workaround:

  1. Open the Outlook mobile app on iOS or Android.
  2. Tap your profile picture or initials in the top-left corner.
  3. Tap the Settings gear icon at the bottom of the menu.
  4. Scroll down and tap your email account.
  5. Look for Storage or Mailbox usage. On some versions, this option appears under “Account” or “Sync settings.”
  6. If you don’t see it, switch to the web version in your mobile browser for the full storage view.

Microsoft updates the mobile app frequently, so the exact location of this setting might change. If you can’t find it, just use the web method on your phone—it’s the same data.

Method 4: Using PowerShell (For Advanced Users)

If you’re an IT admin or a power user, you can check mailbox size with PowerShell commands. This method is great for checking multiple mailboxes at once. Here’s a simple script:

  1. Open Windows PowerShell as an administrator.
  2. Connect to Exchange Online by running: Connect-ExchangeOnline -UserPrincipalName your-email@domain.com
  3. Enter your admin credentials when prompted.
  4. Run this command to check a single mailbox: Get-MailboxStatistics -Identity "user@domain.com" | Format-List TotalItemSize, ItemCount
  5. The output shows the total size in bytes and the number of items. You can convert to GB by dividing by 1,073,741,824.

PowerShell gives you the most accurate numbers, including the Recoverable Items folder size, which the GUI methods sometimes hide. But it requires some technical know-how. If you’re not comfortable with command lines, stick with the desktop or web methods.

What To Do When Your Mailbox Is Too Large

Once you know your size, you might find it’s close to the limit. Don’t panic. Here are practical steps to free up space:

  • Empty the Deleted Items folder. Right-click it and choose “Empty Folder.” This alone can recover several GB.
  • Sort your Inbox by size. In Outlook, click the “Size” column header to see the largest emails first. Delete or archive the big ones.
  • Remove large attachments. Save attachments to OneDrive or a local folder, then delete the email. Or use the “Clean Up” tool in Outlook to remove redundant messages.
  • Archive old emails. Use the built-in AutoArchive feature or manually move old messages to a PST file. Archiving moves them out of your online mailbox but keeps them accessible.
  • Check the “Recoverable Items” folder. In Outlook on the web, go to Settings > View all Outlook settings > Mail > Recoverable items. Delete anything you don’t need.
  • Request a larger mailbox from your IT admin. Some plans allow quota increases up to 100 GB or more.

Regular maintenance prevents the problem from coming back. Set a reminder to check your mailbox size every month. If you send or receive many large files, check weekly.

Common Mistakes When Checking Mailbox Size

Even experienced users make errors. Here are the most frequent pitfalls:

  • Forgetting to include the Recoverable Items folder. This folder holds deleted items that are still recoverable, and it counts toward your quota. The desktop app’s “Mailbox Size” window shows it separately, but many people miss it.
  • Relying only on the Outlook status bar. The bottom bar shows “X items, Y MB,” but that number is often outdated or inaccurate. Always use the proper mailbox size tool.
  • Not refreshing after deleting items. If you delete a large email, the size doesn’t update immediately. Close and reopen the mailbox size window, or wait a few minutes.
  • Confusing “mailbox size” with “OST file size.” The OST file on your computer is a cached copy, not the actual server size. The server size is what matters for your quota.

Avoid these mistakes, and you’ll always have an accurate picture of your storage.

How To Check Mailbox Size For Shared Mailboxes

If you have access to a shared mailbox (like info@company.com or support@company.com), you can check its size too. The process is slightly different:

  1. In the Outlook desktop app, go to File > Account Settings > Account Settings.
  2. Select the shared mailbox account (it might appear under your primary account).
  3. Click Change > More Settings > Advanced > Mailbox Size.
  4. If the shared mailbox doesn’t appear, you might need to add it manually. Go to File > Open & Export > Other User’s Folder, then type the mailbox name.

On the web, shared mailboxes appear in your folder list. Right-click the shared mailbox name and choose “Properties,” then look for “Mailbox size” or “Storage.” Some organizations restrict this view, so you might need admin help.

Using Third-Party Tools To Monitor Mailbox Size

Some people prefer automated tools that send alerts when the mailbox gets full. Tools like Mailbox Size Monitor or ManageEngine can track multiple mailboxes and send email reports. However, for most individuals, the built-in methods above are sufficient.

Be cautious with third-party tools. Only use ones from trusted sources, and never give them your password. Microsoft’s own tools are always the safest bet.

Frequently Asked Questions (FAQ)

1. How often should I check my Outlook 365 mailbox size?

It depends on your email volume. If you get 50+ emails daily with attachments, check every two weeks. For light users, once a month is fine. Set a recurring calendar reminder.

2. What is the maximum mailbox size in Outlook 365?

Most standard plans give 50 GB. Some enterprise plans offer 100 GB. If you need more, your IT admin can request an increase or enable an archive mailbox with unlimited storage.

3. Why does my mailbox size show different numbers in desktop vs web?

The desktop app caches data, so it might be a few hours behind. The web version shows the live server data. For the most accurate number, use Outlook on the web.

4. Can I check mailbox size without opening Outlook?

Yes. Use Outlook on the web from any browser. You can also use the Microsoft 365 admin center if you’re an admin, or run a PowerShell script from your computer.

5. Does deleting emails from my phone free up server space?

Yes, but only if you also empty the Deleted Items folder. Deleting from your phone moves emails to the Deleted Items folder, which still counts toward your quota until you empty it.

Final Tips For Managing Your Outlook 365 Mailbox

Knowing how to check mailbox size in Outlook 365 is just the beginning. Combine this knowledge with good email habits. Unsubscribe from newsletters you never read. Use rules to automatically move large emails to a separate folder. And always empty your Deleted Items folder at the end of each week.

If you find yourself constantly running out of space, consider enabling an online archive. This feature moves old emails to a separate mailbox that doesn’t count toward your main quota. You can set it up in Outlook on the web under Settings > Mail > Automatic archiving.

One last tip: don’t ignore the “mailbox almost full” warning. When you see it, take action immediately. Waiting even a day can cause you to miss critical emails. With the steps in this guide, you’ll never be caught off guard again.

So go ahead and check your mailbox size right now. It takes less than two minutes, and it could save you from a lot of frustration later. Your future self will thank you.