Knowledge Base / MS Office

Outlook: Increase the Maximum PST File Size

Updated November 11, 2026

Overview

Outlook limits PST and OST data files to 50 GB by default. When a file reaches that size, Outlook stops adding items to it. You can raise the limit by adding two values to the registry. This guide raises it to 100 GB and shows two ways to do it: Registry Editor, or PowerShell.

Two values control the limit. Both are measured in megabytes, so 100 GB is entered as 102400 (100 x 1024).

• MaxLargeFileSize is the hard limit. Outlook will not let the file grow past it.
• WarnLargeFileSize is the size at which Outlook starts warning you. Set it to about 95% of the maximum.

Applies To

This article is for Outlook 2016 and later only: Outlook 2016, 2019, 2021 and the Microsoft 365 desktop app (the registry version number for all of these is 16.0). It covers the current user only, not other users on the PC. It does not apply to the new Outlook for Windows, which does not use PST files.

Before You Start

• Close Outlook completely, and check Task Manager to make sure OUTLOOK.EXE is not still running.
• Back up the registry key if it already exists (File > Export in Registry Editor), or create a restore point.
• You do not need administrator rights, because the change is made under HKEY_CURRENT_USER.

Method 1: Registry Editor

1. Press Win + R, type regedit, and press Enter.
2. Go to HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\PST.
3. If the PST key does not exist, right-click the Outlook key, choose New > Key, and name it PST.
4. With the PST key selected, right-click an empty area of the right pane and choose New > DWORD (32-bit) Value. Name it MaxLargeFileSize.
5. Double-click MaxLargeFileSize, set Base to Decimal, enter 102400, and click OK.
6. Create a second DWORD (32-bit) Value named WarnLargeFileSize. Double-click it, set Base to Decimal, enter 97280 (95% of 102400), and click OK.
7. Close Registry Editor and start Outlook.

Note: if you leave Base on Hexadecimal, 102400 is read as a different number. Always choose Decimal before typing the value.

Method 2: PowerShell

Open a regular (non-administrator) PowerShell window with Outlook closed. First set the key path and create the key if it is missing:

$key = "HKCU:\Software\Microsoft\Office\16.0\Outlook\PST"
if (-not (Test-Path $key)) { New-Item -Path $key | Out-Null }

Then add the two values (this also overwrites them if they already exist):

New-ItemProperty -Path $key -Name MaxLargeFileSize -PropertyType DWord -Value 102400 -Force | Out-Null
New-ItemProperty -Path $key -Name WarnLargeFileSize -PropertyType DWord -Value 97280 -Force | Out-Null

Confirm the values were saved:

Get-ItemProperty -Path $key | Select-Object MaxLargeFileSize, WarnLargeFileSize

The output should show 102400 and 97280. Start Outlook again afterwards.

Other Sizes

To use a different limit, multiply the size in GB by 1024 for MaxLargeFileSize, then take 95% of that for WarnLargeFileSize:

• 50 GB (default): 51200 and 48640
• 100 GB: 102400 and 97280
• 150 GB: 153600 and 145920

Undo the Change

To return to the 50 GB default, delete both values in Registry Editor, or run this in PowerShell:

Remove-ItemProperty -Path "HKCU:\Software\Microsoft\Office\16.0\Outlook\PST" -Name MaxLargeFileSize, WarnLargeFileSize

Cautions

• Very large PST and OST files are slower and more likely to become corrupted. Splitting old mail into several smaller archive PST files is safer than one huge one.
• Keep PST files on a local drive. Microsoft does not support them on network shares or in OneDrive-synced folders.
• Back up the PST file before it grows past 50 GB.
• Compacting a file (File > Account Settings > Data Files > Settings > Compact Now) frees space but does not change the limit.
• If your organization manages Outlook with Group Policy, a policy value can override these settings.

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