Why Is Gmail Rejecting a File That's Only 26MB?
Finder or Windows Explorer says the file is 26MB. Gmail's attachment limit is famously 25MB. Close enough that it should probably work, and yet the message bounces. It's not a glitch, and the two numbers aren't actually measuring the same thing.
The limit applies to the encoded size, not the raw file
Email was originally designed to carry plain text, not arbitrary binary data. To get a binary file (a PDF, a video, an image) through that text-only pipe safely, email clients encode attachments using a scheme called base64, which converts binary bytes into a restricted set of plain-text characters that any mail server can pass along without corrupting them.
Base64 does this by taking every 3 raw bytes and representing them as 4 encoded characters. That's a roughly 33% size increase before the message is even sent. A file that's 20MB on disk is already pushing about 27MB once base64-encoded and wrapped into the actual email. Gmail's 25MB limit is checked against that inflated, encoded size, not the file size your operating system reports. So a file that looks comfortably under the limit in Finder can still bounce, because the version that actually travels over the network is a third larger.
"The limit" isn't one number
Every platform sets its own cap, and they're not close to each other:
- Gmail: 25MB per message, combined across all attachments, measured on the encoded size as above.
- WhatsApp: caps shared files (documents, video) well below what a typical Gmail attachment allows, and further compresses video and images by default unless you specifically choose to send as a document/original file.
- Corporate Exchange/Outlook setups: IT departments commonly set attachment limits far lower than Gmail's, 10MB is a typical default on managed corporate mailboxes, precisely because a mail server handling thousands of accounts can't afford everyone sending large files.
A file that clears one platform's limit can bounce on another without you changing anything about the file itself.
The practical fix
Compress before you send, and leave real margin below the stated limit rather than aiming right at it, given the base64 overhead. Which tool to use depends on the file type:
- A large PDF (usually driven by embedded images or scanned pages), use Compress PDF.
- A video file too large to attach, use Compress Video.
- A photo or image you need under a specific size rather than just "smaller," use Compress Image to Size, which lets you target an exact output size directly, useful when you know the hard limit you're working against.
All three run in your browser, so a file you're about to email somewhere doesn't get uploaded to a third-party server first just to shrink it.
Source: Gmail Help — File size limits