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

Outlook silently blocks attachments over 20MB. Image to Base64 reliably brings a string under that limit in a single pass.

Tool:Image to Base64

It happens more often than you'd think: 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.

Use the tool: Image to Base64 — Runs entirely on your device using open web standards.

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. It takes less time than reading this paragraph.

How to bring a string under 20MB

  1. Open Image to Base64 in any modern browser. Nothing installs.
  2. Drag the string onto the drop zone. Multiple files work too; Image to Base64 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 — Image to Base64 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.

Open the tool

Image to Base64 →

Free, no account required, no watermark.

Frequently asked questions

Is there a way to do this from my phone?

Yes — open Image to Base64 in mobile Safari or Chrome and drop the string from your photo library or Files app. The flow is identical to desktop, just with bigger taps.

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.

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.

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.

Related guides

Ready to try it?

Run it in your browser: Image to Base64. Runs entirely on your device using open web standards.

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.