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Email Attachment Size Limit Checker
Gmail's attachment limit is 25 MB, Outlook.com's is 20 MB, and both are measured after encoding — which inflates every attachment by about a third, so a 19 MB file already exceeds a 25 MB limit once it is packaged. This checker does that maths for any file size and every major provider, and lists the daily sending limits too.
Runs entirely in your browser. Nothing you paste is uploaded, logged or stored.
Several files? Add them up — the limit applies to the whole message.
On the wire, after encoding
24.0 MB
Base64 adds about 33%. Your 18.0 MB file travels as 24.0 MB — 5 of 8 providers will send it.
| Provider | Sends | Send limit | Receive limit | Largest file that fits | Over the limit |
|---|---|---|---|---|---|
| Gmail / Google Workspace2,000 recipients/day (Workspace); 500 for a free account | fits | 25.0 MB | 50.0 MB | 18.7 MB | Files over the limit are swapped for a Google Drive link automatically.Source: Google Workspace Admin Help |
| Outlook.com300 recipients/day for a new account, rising with account age | too big | 20.0 MB | 150.0 MB | 15.0 MB | Prompts you to upload to OneDrive and send a link instead.Source: Microsoft Support |
| Microsoft 365 (Exchange Online)10,000 recipients/day per mailbox | fits | 25.0 MB | 25.0 MB | 18.7 MB | The message is rejected with a 552 error. The tenant default can be raised to 150 MB by an admin.Source: Microsoft Exchange Online limits |
| Apple iCloud Mail1,000 messages/day, 500 recipients per message | too big | 20.0 MB | 20.0 MB | 15.0 MB | Mail Drop takes over for attachments up to 5 GB, sending a link valid for 30 days.Source: Apple Support |
| Yahoo Mail | fits | 25.0 MB | 25.0 MB | 18.7 MB | Rejected outright — no automatic link fallback.Source: Yahoo Help |
| Proton MailVaries by plan; free accounts are limited to 150 messages/day | fits | 25.0 MB | 25.0 MB | 18.7 MB | Rejected. Proton Drive sharing is the workaround.Source: Proton support |
| Zoho Mail | too big | 20.0 MB | 40.0 MB | 15.0 MB | Rejected above the plan limit; larger allowances exist on higher tiers.Source: Zoho Mail help |
| Fastmail | fits | 70.0 MB | 70.0 MB | 52.5 MB | One of the highest limits available — but the receiving server's limit still applies.Source: Fastmail help |
Both limits apply: yours on the way out, the recipient’s on the way in, and the lower one wins. Figures checked September 2026 — providers change them, so confirm before sending something critical.
- Enter a file size and see which providers will accept it
- Applies the base64 overhead that trips up every borderline attachment
- Send limits and receive limits shown separately — they differ
- Daily sending limits per provider, for bulk and cold-mail planning
- What each provider does when you go over: link, or hard rejection
- The largest file that fits each limit, worked out for you
How to use it
Four steps, about thirty seconds
- 01
Enter your file size
In MB, as your operating system reports it. Several files? Add them up first — the limit applies to the whole message.
- 02
Read the verdict per provider
Green means it fits after encoding. The table shows both the sending limit and the recipient's receiving limit.
- 03
Check the recipient's side too
Your provider's send limit is only half of it. Microsoft 365 defaults to 25 MB on receive, which is where most 'but it sent fine' failures happen.
- 04
Go around it if needed
Over the limit, a link beats a fight: Drive, OneDrive, Mail Drop or a signed URL. The page explains which each provider does automatically.
The encoding
Why a 24 MB file fails a 25 MB limit
Email is a text protocol. It has been since 1982, and it cannot carry raw binary data — so every attachment is base64-encoded, which represents three bytes using four characters. The result is about 33% larger than the file on your disk, and the encoded size is the one the provider checks.
So the working rule is: your real ceiling is roughly three-quarters of the stated limit. An 18 MB file fits comfortably under Gmail’s 25 MB. A 20 MB file is borderline once the headers and MIME framing are added. A 24 MB file — under the limit by every number your file manager shows you — is rejected.
Two limits
Yours on the way out, theirs on the way in
The second most common failure has nothing to do with encoding: the message left fine and the recipient’s server refused it. Every hop can enforce its own ceiling, and the lowest one decides.
This is why a 25 MB attachment sent from Gmail bounces into a Microsoft 365 tenant that never raised its 25 MB default — the encoding pushed it past the line on the receiving side. It is also why “it worked when I sent it to myself” proves nothing at all.
The bounce, when it comes, is usually a 552 (message too large) or 5.3.4 (message size exceeds fixed maximum). Both name the limit that was hit; the header analyzer will show you which server said it.
Going around it
What to do when the file is too big
Ranked by how well each option actually works, rather than by how obvious it is.
| Option | How it goes | Watch out for |
|---|---|---|
| A cloud link | Best by a distance. Gmail swaps in a Drive link automatically over 25 MB; Outlook.com offers OneDrive; Apple Mail uses Mail Drop for up to 5 GB. | Check the sharing permissions. A link the recipient cannot open is worse than a bounce, because nobody finds out for a day. |
| Compress it | Useful for documents and uncompressed formats. A folder of Word files can shrink dramatically. | Nothing on a photo, video or PDF — those are already compressed. And many providers block .zip files containing executables outright. |
| Shrink the original | Often the real answer. A PDF exported at print resolution is frequently ten times larger than it needs to be. | Re-exporting at screen resolution can lose detail someone needs. Ask before flattening a document. |
| Split across emails | Works, technically. | The worst option. It irritates the recipient, multi-part archives fail if one message lands in spam, and a burst of near-identical attachments is itself a spam pattern. |
Sending limits
The other limit: how many, not how big
Daily sending caps are anti-abuse ceilings, not targets. Google Workspace allows 2,000 recipients a day (500 on a free Gmail account), Microsoft 365 allows 10,000 per mailbox, Outlook.com starts a new account near 300 and raises it with age, and iCloud allows 1,000 messages.
Two things people get wrong here. First, recipients, not messages — one email to fifty people spends fifty of your allowance. Second, hitting the cap is not the real risk; approaching it from a new domain is. A brand-new sending domain going from zero to a thousand messages a day looks exactly like a compromised account, and the response is usually a quiet routing of your mail to spam rather than a visible block.
If you are planning volume, the outbound calculator works out how many sends your reply rate actually requires — which is often far fewer than the cap, and the useful number to plan against.
Questions
Frequently asked
Short answers, including the ones where the honest answer is “this tool can’t do that”.
25 MB for a message you send, and 50 MB for one you receive. Files above 25 MB are replaced automatically with a Google Drive link rather than rejected.
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