Email Attachment Size Limits, and Why 25 MB Is Really 18

Most people find out about email attachment limits the hard way: you attach a video, hit send, and get a bounce message or a polite suggestion to “use Drive instead”.

Here are the limits for the big providers, plus the bit almost nobody mentions: you can’t actually attach a file as big as the limit says.

The advertised limits

Provider	Attachment limit
Gmail	25 MB per email
Outlook.com / Hotmail	20 MB per email
Yahoo Mail	25 MB per email
iCloud Mail	20 MB per email (more with Mail Drop)
Company email (Exchange, Google Workspace)	Whatever your IT team set, sometimes as low as 10 MB

Providers occasionally change these, so treat them as a rough guide. The more important thing is that the limit usually counts the whole message, not just your file.

Why a 25 MB limit won’t take a 25 MB file

Email was designed for text. To send a photo or a PDF, your mail app converts the file into plain text characters using something called Base64 encoding. Base64 turns every 3 bytes of your file into 4 characters of text, so the attachment grows by roughly a third in transit.

The maths:

• A 25 MB limit ÷ 1.33 ≈ 18.7 MB of actual file.
• A 20 MB limit ÷ 1.33 ≈ 15 MB of actual file.

Add the message body, your signature and a couple of email headers, and the safe size is a little lower again. If a file is anywhere near 18 MB, assume Gmail will push back.

There’s a second limit people forget about: the receiver’s inbox. You might be able to send 25 MB, but if the person you’re emailing is on a company server that only accepts 10 MB, the message bounces on their side.

What the “send as a link” option does

When you go over the limit, Gmail offers to upload the file to Google Drive and put a link in the email. Outlook does the same with OneDrive, and Apple has Mail Drop.

This works fine, with a few catches:

• The file now lives in your cloud storage and counts against your quota, until you remember to delete it.
• The upload has to finish before the email can go, which is slow for big files on home internet.
• Sharing permissions can trip people up. “I can’t open it, it says I need access” is a classic.

Better options for anything over ~15 MB

For one-off sends, skip the attachment entirely and send a link to the file:

1. Compress it if it’s documents. A folder of Word files or spreadsheets can shrink a lot in a ZIP. Photos, videos and PDFs barely shrink, so don’t expect miracles there.
2. Use a direct transfer tool. With BoltShare you pick the file, copy the link, and paste it into your email like any other link. When the recipient opens it, the file downloads straight from your device. Nothing is uploaded first, and nothing sits in a cloud folder afterwards.
3. Use cloud storage when the recipient needs it later. If they won’t open the email for days, a direct transfer won’t work, because your computer needs to be online when they download. Shared cloud storage is the better choice there. We wrote more about this trade-off in upload services vs direct transfer.

A quick habit that saves bounces

Before attaching anything large, check the file size (right-click → Properties on Windows, or Get Info on a Mac). Under 15 MB, attach away. Over that, send a link. That’s the whole rule, and it avoids most “message undeliverable” emails.