Outlook Guide

How to Merge PST FilesOutlook's Import wizard for a few, one combined PST for many

Merge PST files two ways: Outlook's Import wizard with the right duplicate setting, or combine dozens of PST files into one in a single run. Limits inside.

By 📅 Updated: September 7, 2026 ⏲️ 9 min read ✓ Verified against Microsoft's published documentation 🖼️ 4 screens

To merge PST files in Outlook, open the target data file, then use File, Open & Export, Import/Export, Import from another program or file, Outlook Data File (.pst). Pick the PST to fold in, choose Do not import duplicates, tick Include subfolders and select Import items into the same folder in the target file. Repeat for each PST, then run Compact Now. For a stack of files, a leaver's five archives or a decade of yearly PSTs, a converter can combine PST files from a whole folder into one PST in a single run without Outlook. Keep the result under 25 GB so Outlook stays quick, and under 20 GB if it is bound for Office 365.

PST files multiply, and sooner or later someone needs to merge Outlook PST files back into one. One per year of archiving, one per PC a person has owned, one per export someone ran for a project, five from an ex-employee's desktop that the manager now has to read. Outlook has no merge button, but its Import wizard can pour one PST into another. Done carefully, that is a real merge. Done carelessly it doubles every message. This guide shows the wizard route with the settings that matter and the size limits that decide when merging is a bad idea. It ends with the bulk route for when clicking through file after file is not an option.

Before You Merge PST Files

Time30 min to a day
DifficultyEasy
CostFree with Outlook
Runs onWindows and classic Outlook

How many files, and where is the result going? Those two answers pick the method.

Two to five PST files into one Yearly archives or an old laptop's data file. Outlook's Import wizard handles it in an afternoon, one file at a time. Use the Outlook wizard → A folder of PST files from a leaver or an export Five archives from a departed colleague, or forty files a migration tool left behind. Combine them in one run and get one file per person. Use the bulk route → The result is going to Office 365 or a shared drive A merged 40 GB file is the wrong shape for the Import service and slow for Outlook. Sometimes the answer is not to merge, or to merge into parts. Read when not to merge →
Four things before the first import
  • Copies of every PSTThe wizard reads the source and writes into the target, but a crash mid-import can leave the target in a mess. Copy everything to another drive first.
  • Classic Outlook on WindowsThe Import and Export wizard lives in every classic Outlook release from 2013 through Outlook for Microsoft 365. PST import arrived in new Outlook in 2026, but it works only with classic Outlook installed and a Microsoft 365 subscription.
  • A target PST that is new or known goodMerge into a fresh Outlook Data File rather than into a ten year old one. Run ScanPST on any source that has ever shown an error.
  • A size budgetAdd up the sources. If the total is over 25 GB, plan parts instead of one file. If it is bound for Office 365, the parts must be under 20 GB each.
Five words this guide leans on
Outlook Data File
Outlook's menu name for a PST. Created from New Items, More Items, Outlook Data File; an existing one is attached from File, Open Outlook Data File.
Import wizard
File, Open & Export, Import/Export. The only built-in way to move the contents of one PST into another.
Do not import duplicates
The wizard option that skips items already present in the destination. The setting that turns an import into a merge rather than a doubling.
Compact Now
Account Settings, Data Files, Settings, Compact Now. Shrinks the file after items are moved or skipped. Without it the merged PST reports the wrong size.
PST merge
Combining several PST files into one with the folder tree kept. Outlook has no single command for it, so the job is the wizard or a converter.

Merge PST Files in Outlook in 5 Steps

The wizard imports one PST into another, which is how you merge Outlook PST files without a dedicated tool. Run it once per source file, always into the same target. That is what turns it into a merge.

1

Open the target PST file and back everything up

Decide which file receives everything. For yearly archives make a new one: Home, New Items, More Items, Outlook Data File, saved as something like combined-archive-2016-2020.pst. For a leaver's files the target is often the manager's own archive PST, attached through File, Open & Export, Open Outlook Data File. Copy every source PST to a backup folder before you start.

You should seethe target PST in the folder pane and the sources still on disk, untouched. The wizard reads the sources; it does not need them attached.

2

Start Outlook's Import and Export wizard

Go to File, Open & Export, Import/Export. Choose Import from another program or file, then Outlook Data File (.pst) as the file type. Click Next.

Classic Outlook · Import and Export WizardScreen 1 of 4
Swipe sideways to see the whole screen

Step 2Import from another program or file. The next page asks for Outlook Data File (.pst).

3

Pick the PST to merge and set the duplicate rule

Browse to the first source PST. Then the option that decides everything: choose Do not import duplicates. Allow duplicates to be created is how people end up with two of every message. Replace duplicates with items imported is for restoring a newer copy over an older one, and in a merge of different years it does nothing useful while making the file larger. The wizard's duplicate check is imperfect: items that an earlier export or another tool rewrote may not be recognised, so expect a few doubles when sources overlap.

Classic Outlook · File and duplicatesScreen 2 of 4
Swipe sideways to see the whole screen

Step 3Do not import duplicates. The one setting that makes this a merge.

4

Choose the PST folders and the destination

Select the top of the source PST so everything is included and tick Include subfolders. Use Filter only when you want part of a file, such as one year of a bigger archive. Then choose Import items into the same folder in and pick the target PST from the list. This keeps Inbox with Inbox and Sent Items with Sent Items across the merged file. Import items into the current folder is for dropping one PST inside a subfolder of the target instead, which is a tidy way to keep a leaver's mail separate under their name. Click Finish.

You should seea progress bar and then the source's folders appearing inside the target with their items. Open one folder and confirm dates and attachments are intact.

Classic Outlook · Folders and destinationScreen 3 of 4
Swipe sideways to see the whole screen

Step 4Include subfolders, then Import items into the same folder in the combined archive.

5

Repeat for each PST, check the counts and compact

Run the wizard again for every remaining source, always into the same target with the same duplicate setting. When the last one finishes, compare folder item counts: right click a folder, open Properties and add the source counts against the target. Then File, Account Settings, Account Settings, Data Files, select the target, Settings, Compact Now. Keep the source files until you have used the merged one for a few weeks.

Windows Explorer · Before and afterScreen 4 of 4
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ResultFive files in, one combined PST out, smaller than the sum once duplicates were dropped.

How Big Is Too Big for a Merged PST File

Merging trades many files for one big one, and Outlook has opinions about big files. Microsoft's own guidance puts the comfort zone well below the format's ceiling.

Merged PST sizeWhat Microsoft saysWhat it means for the merge
Up to 5 GBGood experience on most hardwareMerge freely
5 to 10 GBShort pauses possible, hardware dependentFine on an SSD
10 to 25 GBShort pauses on most hardwareMerge into two or three files by year instead
25 GB and aboveFrequent pausesDo not merge; keep parts
50 GBDefault Unicode ceilingMaxLargeFileSize can raise it; Microsoft does not recommend doing so
2 GB (ANSI)Hard limitNever merge into an ANSI file; import it into a Unicode target instead

Two more thresholds bite before the file size does: around 100,000 items in one folder and around 10,000 folders in one file both slow Outlook down. A merge of yearly Inboxes into one Inbox crosses the first one fast, which is a reason to keep year folders rather than pouring everything into a single Inbox.

Combine PST Files in Bulk Without Outlook

The wizard is one file per run, with Outlook open and busy for the duration. For a leaver's five archives it is fine. For the forty PST files a migration tool or an Enterprise Vault export left on a share, or for twenty leavers at once, you want to merge PST files in bulk rather than one wizard run at a time. Univik Email Archive Converter reads PST directly, so Outlook is not needed on the machine.

  1. Add the folder of PST filesAdd Folder picks up every PST inside, including ANSI files and files Outlook is not attached to, with their folder structure.
  2. Choose one PST outputPick Outlook PST as the format and merge the sources into a single PST in Unicode, with each source kept as its own top-level folder or blended by folder name. For several people, group by mailbox so each person gets one combined file.
  3. Set a part size if the result would be largeA 60 GB collection becomes numbered parts under 20 GB for Office 365, or 25 GB parts for Outlook, rather than one unusable file.
  4. Read the logEvery message written is logged against its source file, which is the count check the wizard never gives you.
Combine a folder of PST files into one Unicode PST in a single run

Univik Email Archive Converter reads PST files without Outlook, merges them into one file or one file per mailbox, splits at whatever size you choose and writes a per-message log. Runs offline on Windows.

⬇️ Download Free Trial See All Features →

Windows 11 / 10 · Outlook not required · the trial merges a sample from each file so you can open the combined PST first

When Not to Merge PST Files

Bound for Office 365
The Import service wants PST files under 20 GB and writes them all into the same mailbox anyway. Import the parts with one mapping file; the merge happens in the archive mailbox. See how to import PST to Office 365.
Legal or HR evidence
A merged file loses the boundary between sources. Keep a leaver's files separate, or import each into its own named subfolder, so provenance survives.
Total over 25 GB
Merge into parts by year instead. Outlook stays quick and a damaged part costs one year, not a decade. The split a large PST file guide is the other half of this one.
Sources overlap heavily
Two exports of the same mailbox from different dates will produce doubles the wizard cannot always catch. Keep the newer, or de-duplicate before merging.

PST Merge Method Comparison

MethodCostFiles per runNeeds OutlookDuplicate handlingCount check
Outlook Import wizardFreeOneClassicDo not import duplicates, imperfectManual
Drag folders between PST filesFreeOne folder at a timeClassicNoneManual
Univik Email Archive ConverterTrial, then licenceA whole folderNoPer-source folders keep provenancePer-message log

After the PST Merge

The merge worked when
  • Every source folder's item count is accounted for in the combined PST, minus the duplicates you chose to skip
  • Compact Now has run and Explorer shows a file size close to the sum of the sources, not double it
  • A message from the oldest source opens with its attachment and original date, and search finds it by a word in the body

Detach the source PST files from Outlook but keep them on disk for a month. If the merged file is for a leaver's manager, attach it to their profile and name the top folder after the person. If it is going to Office 365, stop here and use the parts instead, as described above.

Fix Common PST Merge Problems

Three problems follow most PST merges. Red is the symptom, green is what to do.

1Every message appears twice after the import

Do this: Allow duplicates was selected, or the source was a rewritten export the wizard could not match. Delete the imported folders and run the import again with Do not import duplicates. Or import into a separate subfolder and let a de-duplication pass clean it up.

2Outlook reports the maximum size reached mid-import

Do this: The target is an ANSI PST at 2 GB, or a Unicode file at the 50 GB ceiling. Create a fresh Unicode Outlook Data File as the target and import into parts by year.

3The merged PST is huge but Explorer shows the old size or Outlook is slow

Do this: Compact Now has not run, or the merged file is past 25 GB. Compact first. If it is still over 25 GB, split it into year parts; Outlook's pauses come from size, not from the merge.

Have a PST collection from a leaver or an export folder and not sure whether to merge or split? Send our support team the file list with sizes and where the result is going. We will suggest one file or parts.

PST merge questions

How do I merge two PST files in Outlook?
Open the target PST in classic Outlook, then File, Open and Export, Import/Export, Import from another program or file, Outlook Data File (.pst). Choose the second PST, select Do not import duplicates, tick Include subfolders and choose Import items into the same folder in the target. Finish, then run Compact Now on the target.
Can I combine PST files without creating duplicates?
Mostly. The Import wizard's Do not import duplicates option skips items already present, but it does not recognise items that another export rewrote, so overlapping sources can still produce a few doubles. Merging sources that cover different date ranges avoids the problem.
Is there a free way to merge PST files?
Yes, Outlook's Import and Export wizard, one file per run. It is the only built-in route; Microsoft ships no separate PST merge tool. For many files at once, a converter that combines a folder of PST files into one is the time saver.
How big can a merged PST file be?
The Unicode format allows 50 GB by default in Outlook 2010 and later, but Microsoft reports short pauses above 10 GB and frequent pauses above 25 GB. Merge into parts by year when the total passes 25 GB, and keep parts under 20 GB when the Microsoft 365 Import service is the destination.
Can I merge PST files in new Outlook?
New Outlook added PST import in 2026, but it still needs classic Outlook on the same PC and a Microsoft 365 subscription. Classic Outlook's Import and Export wizard remains the dependable route for merging.
Should I merge PST files before importing to Office 365?
No. Keep each PST under 20 GB and list them all in one mapping file pointing at the same mailbox. The Import service writes them into the same archive mailbox, which is the merge you wanted, without the risk of one oversized file.
About This Guide

Written and maintained by and the Univik team, developers of Windows data conversion and recovery software since 2013. The wizard labels, duplicate options and size guidance in this guide were checked against Microsoft's current Outlook documentation and Microsoft Q&A answers from Microsoft staff. Last verified September 7, 2026. Need a hand? Contact our support team.

Sources checked: Microsoft Support, Import email, contacts and calendar from an Outlook .pst file · Microsoft Learn, Configure the size limit for PST and OST files · Microsoft Learn, Outlook pauses if you have a large data file · Microsoft Q&A, How do I merge two Outlook PST files

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