Free OST to PST Converter
Turn the offline OST file Outlook keeps for Exchange and Microsoft 365 accounts into a PST you can open, archive or move — without sending your mail to anyone. The conversion engine runs inside your browser tab and reads and writes only your local disk.
Your OST never leaves this computer. The converter runs on a separate page with no analytics, in a worker that cannot make network requests, and writes the PST to your own disk.
How to convert an Outlook OST file to PST
- Copy your OST. Close Outlook, then copy the .ost file (usually in %LOCALAPPDATA%\Microsoft\Outlook). The converter only reads it, but keep the original.
- Open the workspace. Open the converter workspace. It loads only its own files, so you can disconnect from the internet once it says Ready.
- Choose the OST. Pick or drop the file. The workspace reads its header and index and shows how many folders and items it holds.
- Convert and check. Convert. Folders, mail, attachments, contacts, calendar and tasks are rebuilt as a Unicode PST, then every property and table row is compared with the OST.
- Save the PST. Save the .pst and open it in Outlook with File → Open & Export → Open Outlook Data File.
Use Cases
Frequently Asked Questions
Is my mailbox uploaded anywhere?
No. The workspace is a separate page with no analytics or third-party scripts; its security policy lets it talk only to this site, and the conversion worker that reads your OST is not allowed to make any network request at all. The OST is read from your disk, and the PST is staged in your browser's private storage until you save it or leave the page (anything left behind is cleared on your next visit).
Which OST files are supported?
Unicode OST files from Outlook 2013, 2016, 2019, 2021 and Microsoft 365. OST files from Outlook 2003–2010, ANSI OST files from Outlook 2002 and earlier, and OSTs protected with Windows Information Protection are recognised and refused with a clear message.
Will Outlook open the PST?
The output is a standard Unicode PST (format version 23) with fresh indexes, valid allocation maps and the folder layout a PST needs, so it is meant to open directly with File → Open & Export → Open Outlook Data File — no SCANPST repair step. This is an alpha release: keep your original OST until you have opened the PST and checked your mail.
How is the result checked?
Before you can save, the workspace re-reads the PST it wrote and checks its structure (checksums, indexes, allocation maps, heaps and tables) and compares every folder, item, property, table row and attachment with the OST. The only differences allowed are the adjustments a PST needs — folder layout, store settings, table cells refreshed from the items they list, folder counts, and OST-only caches left out — and the report lists them. Anything else stops the conversion.
What if the OST is damaged?
The converter is strict: if a checksum or structure in the OST does not add up, it stops and says so rather than producing a PST with missing or wrong data. Repairing damaged OST files is not part of this tool.
How large a file can it handle?
Attachments and message bodies are streamed from disk to disk, so the mailbox size itself is not the limit — synthetic mailboxes of several gigabytes convert fine. What is held in memory is one folder's list at a time: a folder of 200,000 items needs about 2 GB of RAM, and folders much larger than that may not fit in a browser tab. You also need free disk space of a few times the OST size, since PST blocks are not compressed. Very large real mailboxes are still being tested.
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