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string won't attach to Outlook? Bring it under the 20MB cap fast

Outlook silently blocks attachments over 20MB. Base64 Encoder / Decoder reliably brings a string under that limit in a single pass.

Tool:Base64 Encoder / Decoder

Most people hit this exact problem at least once: a string just over the Outlook attachment limit, and you have to send it now.

Outlook's hard cap is 20MB per outgoing message. Outlook.com caps at 20MB; the Microsoft 365 desktop client allows 33MB for outgoing mail. Anything bigger gets rejected — sometimes silently, more often after you've waited 30 seconds for the upload bar to crawl. The fix is to bring the string below that threshold before you hit Attach.

Try it now: Base64 Encoder / Decoder — Free, no account required, no watermark.

Why this happens

Strings grow for predictable reasons — embedded images at full camera resolution, fonts shipped twice, scanned pages saved at 600 DPI, video clips that were never meant for email. The original was fine for archiving, but it isn't shaped for email. Open the tool below and follow along.

How to bring a string under 20MB

  1. Open Base64 Encoder / Decoder in any modern browser. Nothing installs.
  2. Drag the string onto the drop zone. Multiple files work too; Base64 Encoder / Decoder processes them in a single pass.
  3. Pick a compression preset. "Balanced" is the right answer 95% of the time — visually identical output, file size cut by 50–80%.
  4. Wait for processing — usually under five seconds for a string smaller than 50MB.
  5. Download the result. It lands in your default downloads folder under the original filename, suffixed.
  6. Attach the smaller version to Outlook and send. The size badge in the attachment row should now read well below 20MB.

If you have a stack of strings to send to the same person, drop them in together — Base64 Encoder / Decoder handles a batch in one click, then you attach the whole folder.

What if it's still too big?

A few strings resist compression — usually because they're already aggressively compressed, or they're video / audio at high bitrate. Two reliable next moves:

  • Trim or split. If the string is content-rich, sending half today and half tomorrow often beats forcing it into one attachment.
  • Switch to a cloud link. Outlook itself will offer "2GB via OneDrive link" once you exceed the limit. Google Drive, OneDrive, Dropbox, and WeTransfer all give you a copy-paste link that bypasses every mail provider's cap.

Run it in your browser

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Frequently asked questions

What's the maximum I should attach to Outlook, in practice?

Stay 10–15% under the hard cap. Outlook's 20MB is the wire limit, but base64 encoding inflates the body by ~33%. Headroom prevents the "rejected after 30 seconds" failure mode.

Will the recipient be able to tell the string was compressed?

Usually not. The "Balanced" preset on Base64 Encoder / Decoder targets visually indistinguishable output. Only a side-by-side pixel comparison would reveal the difference, and recipients almost never do that.

Why does Outlook reject files over 20MB?

It's a server-side rule, not a client setting. Outlook.com caps at 20MB; the Microsoft 365 desktop client allows 33MB for outgoing mail.

Why is base64 encoding mentioned — what does that mean for me?

Email attachments are base64-encoded on the wire, which adds about 33% to the file size during transit. Outlook's 20MB is measured after that inflation, which is why a string that's exactly 20MB on disk will sometimes still bounce.

Related guides

Ready to try it?

Use the tool: Base64 Encoder / Decoder. Browser-only. Nothing is sent to a server.

Last reviewed May 2026. File-size limits, portal requirements, and software defaults change over time — always verify with the destination platform before uploading time-sensitive documents. References to third-party services and products are for descriptive purposes only and do not imply any partnership or endorsement.