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Free Email Attachment Size & Limit Calculator

This calculator estimates how large your email really is after attachments are Base64 and MIME encoded, then checks it against the Gmail, Outlook.com, Yahoo Mail, or a custom size limit. Encoding adds about 37 percent, so a 20 MB file can bounce from a 25 MB limit. Everything runs in your browser.

Quick examples

Attachments

Enter each file's size as shown by your computer.

Attachment 1 name
Attachment 1 sizeAttachment 1 unit
Attachment 2 name
Attachment 2 sizeAttachment 2 unit

Provider limit

Custom limit (MB)Choose Custom to edit.

Message body and other content (KB)Text, HTML, signature, and inline images.

Estimate

conservativetypical

Size units

Encoded size

Fits Gmail (personal)

14.40 MB of 25.00 MB · 10.60 MB headroom

Raw attachment total11,010,048 bytes: 10.50 MB
Base64 payload4 × ceil(bytes ÷ 3), per file: 14.00 MB
Base64 overhead: +3.50 MB
MIME line wrappingCRLF after every 76 characters: +377.27 KB
MIME headers and boundaries: +7.05 KB
Message body and other contentBase64 encoded: +27.37 KB
Total encoded message15,101,628 bytes: 14.40 MB
Provider limit26,214,400 bytes: 25.00 MB
Remaining headroom37.2% total overhead: 10.60 MB

Recommended maximum raw attachments

18.24 MB

Largest combined size for 2 files that still fits Gmail (personal) with the current body size and conservative assumptions.

Assumptions

Conservative estimate: 6,144 bytes of top-level headers, 320 bytes of MIME headers per attachment plus its filename twice, a 44-byte boundary per part, and a body that is Base64 encoded.
Encoding: every attachment is Base64 encoded on its own and wrapped at 76 characters with CRLF line endings, as specified in RFC 2045.
Units: 1 MB = 1,048,576 bytes for attachments and limits. If a provider counts decimal megabytes, a 25 MB limit is 25,000,000 bytes; switch units to check.
Enforcement varies: providers may measure raw attachments, the encoded message, or both, and change limits without notice. Workspace, Microsoft 365, and self-hosted admins can set different limits, and the recipient's server applies its own.

Up to 25 MB total attachments. Larger files are offered as Google Drive links. Official Gmail (personal) help page Estimates use 20,480 bytes of raw body content.

How to check an email attachment size limit

Add your attachments

Enter a name and size for each file, choosing KB, MB, or GB. Add or remove rows as needed.

Pick a provider limit

Choose Gmail, Outlook.com, Yahoo Mail, or set a custom limit for your own mail server.

Check the encoded size

Review the Base64 and MIME overhead, total encoded estimate, remaining headroom, and recommended maximum raw size.

Why attachments grow: Base64 and MIME overhead

Base64 adds 33 percent

SMTP was designed for text, so binary files are converted to Base64. Every 3 bytes become 4 ASCII characters, and the last group is padded with =. The exact size is 4 × ceil(bytes ÷ 3), so a 3 MB file becomes 4 MB.

Line wrapping adds about 2.6 percent

MIME limits Base64 lines to 76 characters, and each line ends with a 2-byte CRLF. That adds 2 bytes for every 76 encoded characters, bringing the combined encoding overhead to roughly 36.8 percent.

Headers and boundaries

Each attachment gets its own MIME part with Content-Type, Content-Disposition, and Content-Transfer-Encoding headers, separated by boundary lines. The message also carries routing and DKIM headers, and the body itself counts toward the limit.

In practice, divide a provider's limit by about 1.37 to get the largest raw attachment total that is likely to send. For a 25 MB limit that is roughly 18 MB. When a message is too large, the sending or receiving server rejects it, often with an SMTP 552 5.3.4 message size exceeded bounce.

Frequently asked questions

What is the Gmail attachment size limit?

Gmail personal accounts allow up to 25 MB of attachments per message. Because attachments are Base64 encoded, the practical raw limit is closer to 18 MB. Larger files are shared as Google Drive links instead.

What is the Outlook.com attachment size limit?

Outlook.com has a 25 MB file attachment limit. Microsoft 365 and Exchange administrators can set different limits for work accounts, so check with your admin if you send from a company mailbox.

Why does a 20 MB file exceed a 25 MB email limit?

Email is a text format, so binary attachments are Base64 encoded. Base64 turns every 3 bytes into 4 characters, adding about 33 percent, and MIME line breaks add roughly 2.6 percent more. A 20 MB file becomes about 27.4 MB on the wire.

How is the Base64 attachment size calculated?

The exact Base64 length is 4 × ceil(bytes ÷ 3). The encoded text is then wrapped into 76-character lines, and each line ends with a 2-byte CRLF. This calculator applies both steps to every attachment separately.

Are my files uploaded when I use this calculator?

No. You only enter file names and sizes, and every calculation runs locally in your browser. Nothing is uploaded and no API calls are made.

Can the receiving server have a smaller limit?

Yes. The recipient's mail server enforces its own maximum message size, often 10 MB to 50 MB, and admins can change it at any time. A message that fits your provider can still bounce at the destination.