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Arithmetic
Email attachment size calculator
A 24 MB file bounces off a 25 MB limit and everybody blames the email provider. The real reason is encoding, and it is entirely predictable.
The usable figure is always smaller than the advertised one.
Result
The missing third
Email was designed to carry text, so binary attachments are encoded into text before sending, almost always with Base64. That encoding uses four characters for every three bytes, which makes an attachment about 37% larger in transit than it is on disk. Providers apply their limit to the encoded message, not to your original file.
So a 25 MB limit takes a file of roughly 18 MB. A 10 MB limit takes about 7.3 MB. Add the message headers, the body, and any signature image, and the safe figure is a little lower still. This is why the file that is obviously under the limit gets rejected.
If you are over: compress the PDF to a target, split it across two messages, or send a link instead. Compressing a file that is already compressed, a ZIP of JPEGs for instance, gains nothing at all.
Common questions
Does zipping help?
Only for content that is not already compressed, so text, spreadsheets and raw data. Zipping photographs, videos or a PDF full of images saves almost nothing, and the encoding overhead still applies afterwards.
Is the overhead exactly a third?
Base64 itself adds 33.3%, and line breaks in the encoded stream push it to roughly 37%. This page uses the higher figure, because being slightly pessimistic about an email limit costs you nothing.
Why do some providers seem more generous?
Because a few apply the limit to the decoded attachment, and some silently upload large files to their own storage and send a link. You cannot rely on either, especially when the recipient is on a different system with a lower limit than yours.
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