PST to EML: how much bigger does the file get with attachments?
EML stores attachments as base64 text, so every exported message is bigger than the attachments inside it. We exported the three messages of a test archive (attach.pst, 27,444,224 bytes) with the Mailattic engine and measured the result.
| Message | Attachments | Attachment bytes | EML bytes | EML / attachments |
|---|---|---|---|---|
| Files of several types | 7 | 122,300 | 169,665 | 1.39 |
| Large attachment (25 MB) | 1 | 26,214,400 | 35,873,397 | 1.37 |
| Inline image | 1 | 4,000 | 6,114 | 1.53 |
Details
Why it matters: plan disk space before you export a mailbox. A 25 MB attachment becomes a 35.9 MB .eml file, and many mail servers refuse to receive a message that big if you send it back.
Measured on one generated archive. Ratios shrink toward 1.37 as attachments grow because headers and the text body weigh less. The last one is base64 (4 bytes out for 3 in, plus a line break every 76 characters). Tested 2026-10-31. mailattic-core 1.0.0.
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About Mailattic
Mailattic is our Windows app that opens a .pst without Outlook, reads attachments, and exports EML, MBOX and PDF. Read-only: your archive is never changed.
Not affiliated with, endorsed by or connected to Microsoft. Outlook and Microsoft are trademarks of Microsoft Corporation and are used only to say which files this opens.