Outlook Attachment Size Limit: How to Increase

Updated 7 November 2026 · Standard formula, shown below

Yes, you can increase the attachment size limit in Microsoft Outlook, but the maximum you can set depends on your version and account type. For Outlook desktop (Microsoft 365 and most standalone versions), the hard limit is 150 MB, while Outlook on the web (OWA) caps attachments at 20 MB for personal accounts and 25 MB for work/school accounts. This article explains exactly where to change the limit, what the real constraints are, and how to work around them when you need to send larger files.

Where the Limit Comes From

Outlook has two separate attachment limits: one set by the client software and one set by the mail server. The client-side limit is the one you can adjust in the registry or Group Policy; it controls how large an attachment Outlook will allow you to attach before showing an error. The server-side limit is enforced by your email provider (Microsoft 365, Exchange, Gmail, etc.) and cannot be changed from Outlook itself. Even if you raise the client limit to 150 MB, your provider will still reject anything over its own cap.

For Microsoft 365 and Exchange Online, the default server limit is 25 MB per message, but administrators can raise it to 150 MB via the Exchange Admin Center. For personal Outlook.com accounts, the server limit is 20 MB. Gmail’s limit is 25 MB, and Yahoo’s is 25 MB. So the practical maximum for most users is 150 MB, but only if your admin has enabled it.

How to Change the Limit in Outlook Desktop

For Outlook 2013, 2016, 2019, 2021, and Microsoft 365 on Windows, the client-side limit is controlled by a registry key. Follow these steps:

Close Outlook completely.
Press Win + R, type regedit, and press Enter.
Navigate to: HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Preferences (use 15.0 for Outlook 2013, 14.0 for 2010).
Right-click on the right pane, select New → DWORD (32-bit) Value.
Name it MaximumAttachmentSize.
Double-click it, select Decimal, and enter the size in kilobytes. For 150 MB, enter 153600 (150 × 1024).
Click OK and restart Outlook.

If the value is set to 0 (or the key is absent), Outlook uses its default limit, which is typically 20 MB for older versions and 150 MB for newer ones. Setting it to any value above 0 enforces that size in KB.

What About Outlook on the Web (OWA)?

Outlook on the web does not have a user-adjustable attachment limit. The 20 MB (personal) or 25 MB (work/school) cap is set by Microsoft and can only be changed by an administrator for business accounts. There is no registry hack or browser setting that overrides it. If you need to send a file larger than that through OWA, you must use a file-sharing service like OneDrive or SharePoint and share a link instead of attaching the file directly.

Common Problems and Error Messages

When you hit the limit, Outlook shows one of these errors:

“The file is too large to attach.” – This is the client-side limit. Apply the registry fix above.
“The attachment size exceeds the allowable limit.” – This is the server-side limit. You cannot fix this from Outlook; you need to reduce the file size or use a link.
“The message size exceeds the maximum allowed.” – This is the total message size (attachments + body + headers), which is often larger than the attachment alone.

A common misconception is that raising the registry value to 150 MB automatically lets you send 150 MB files. It does not—your server must also allow that size. If you get a server-side error, the registry change won’t help.

How to Choose the Right Limit

Before changing the limit, consider the practical constraints:

Recipient’s server: Even if your server allows 150 MB, the recipient’s provider may cap at 25 MB. The message will bounce back.
Email system performance: Large messages slow down sending and receiving, and many corporate servers have lower limits for security reasons.
Alternative methods: For files over 25 MB, using a cloud link is almost always better than increasing the limit.

For most users, setting the client limit to 25 MB or 50 MB is a safe middle ground. If you regularly send large presentations or videos, consider compressing files first or using a file-sharing service.

Alternatives to Increasing the Limit

If you cannot change the server limit, or the recipient’s server is the bottleneck, use one of these methods:

Compress the file: Zip a folder or use a tool like 7-Zip to reduce size. For images, convert to JPEG or reduce resolution.
Cloud storage link: Upload to OneDrive, Google Drive, or Dropbox, then paste a share link into the email. This is the standard solution for large files.
Split the file: Use a tool like WinRAR to split a large archive into multiple parts, each under the limit, and send them in separate emails.
Use a transfer service: Services like WeTransfer (free up to 2 GB) or SendAnywhere are designed for large files, but be aware of privacy implications.

Frequently Asked Questions

Can I increase the attachment limit in Outlook 365?

Yes, for Outlook desktop (Microsoft 365), you can use the registry method above to set the client limit up to 150 MB. However, the server limit for Microsoft 365 is also 150 MB by default, but only if your admin has configured it. For Outlook on the web, you cannot change the limit yourself.

Why can’t I send a 30 MB file even after changing the registry?

Because the registry change only affects the client-side check. The server (Exchange, Gmail, etc.) will still reject the message if its own limit is lower. Check your email provider’s documentation for the maximum message size, and if you are on a work account, ask your IT admin to raise the server limit.

Does the attachment size limit include the email body?

Yes, the total message size includes the body, headers, and all attachments. Outlook’s limit applies to the total message size, not just the attachment. So a 25 MB attachment plus a 1 MB body will be counted as 26 MB.

What is the maximum attachment size for Outlook.com?

Outlook.com (personal) allows attachments up to 20 MB per message. There is no way to increase this from the user side. For larger files, use OneDrive integration to share a link.

Is there a way to send files larger than 150 MB through Outlook?

No, not as a direct attachment. The 150 MB limit is the absolute maximum for Microsoft 365 and Exchange Online. For anything larger, use a cloud storage link or a file transfer service.

Will changing the registry break anything?

No, the MaximumAttachmentSize registry key is a supported setting. However, if you set it too high, you may experience slow sending or server errors. It is safe to revert by deleting the value or setting it to 0.

Method: the standard formula for this calculation, rounded as each result states. See how the calculators work or report an error.