Why did my 24MB attachment fail when Gmail's limit is 25MB?

Because attachments are Base64-encoded before they are sent, which inflates them by about 33%, plus MIME headers. A 24 MB file becomes roughly 33 MB on the wire, which is over the 25 MB limit. The published limit applies to the encoded message, not to the file you selected.

What is the largest video I can actually email?

For Gmail, about 18 MB of actual file. If you do not know what the recipient uses, aim lower still: the Outlook desktop client applies a 20 MB cap, and delivery is governed by the smallest limit anywhere in the chain.

Does this affect Google Drive links too?

No. A Drive link is a few hundred bytes of text in the message body, so encoding overhead is irrelevant. The overhead only applies to files attached to the message itself.

Why a 24 MB video fails a 25 MB limit

Email attachments are Base64-encoded before they are sent, which makes them about33% larger in transit. The published limit applies to that encoded size, not to the file you picked. Gmail's 25 MB therefore means roughly18 MB of actual file.

This is the most common reason a carefully compressed video still bounces, and almost no compression tool accounts for it — including, until recently, this one.

What actually happens to an attachment

Email was designed to carry text. The protocol that moves it between servers is not guaranteed to pass arbitrary binary data through unharmed, so attachments are encoded into a restricted set of ASCII characters using Base64.

Base64 represents every 3 bytes of input as 4 characters of output. That is a fixed ratio: the encoded form is 4/3 the size of the original, about33% larger. Line breaks and MIME part headers add a little more, which is why the practical multiplier is closer to1.37.

The provider's size check happens on the assembled message. By then your file has already grown.

The arithmetic

What each file size becomes on the wire, against Gmail's 25 MB cap.
Your file	Encoded size	Sends?
10 MB	13.7 MB	Yes
15 MB	20.6 MB	Yes
18 MB	24.7 MB	Yes
20 MB	27.4 MB	Rejected
24 MB	32.9 MB	Rejected
25 MB	34.3 MB	Rejected

The rule of thumb: multiply your file size by 1.37 and compare that to the limit. Or work backwards and target about 70% of whatever cap you are aiming at.

It gets worse across providers

Delivery is governed by the smallest limit anywhere in the chain, not by your provider's. You can send 25 MB from Gmail to a recipient whose mail server accepts 10 MB and the message will bounce after you have already sent it.

The Outlook desktop client applies its own 20 MB cap on IMAP and POP accounts, which is lower than the Outlook web interface. The same message can be refused by the app and accepted by the website. Corporate mail gateways commonly sit at 20–25 MB.

If you do not know what the recipient uses, 15 MB of actual file is a target that survives most of the chain.

What this site does about it

The email preset divides by 1.37 before setting a target, so choosing it aims at 17.5 MBof file rather than at the published 25 MB. That is the number that arrives.

Compress a video for email →

When the overhead does not apply

Only attached files are encoded this way. A Google Drive or OneDrive link is a few hundred bytes of text in the message body, so none of this matters — which is exactly why both providers offer to convert oversized attachments into links automatically.

That fallback has its own failure mode: if the file lands in a private folder or an organisation's domain, the recipient gets a permission screen instead of the video, and the upload counts against your storage quota. For anything under18 MB, a real attachment is less likely to go wrong.