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Email Attachment Size Checker

Check email attachment size limits for Gmail, Outlook, Yahoo, Apple Mail, and 12+ providers. Includes a file size calculator with Base64 encoding overhead, blocked file type reference, and tips for sending large files.

About this tool

Every email provider has different attachment size limits, and hitting them causes frustrating bounce-backs. Gmail caps at 25MB, Outlook at 20MB, and when you factor in Base64 encoding overhead (which adds ~37% to your file size), a 19MB file can already be too large for some providers.

This tool lets you check your file size against every major email provider's limits instantly. It accounts for Base64 encoding — the conversion that happens behind the scenes when you attach binary files to email — so you get an accurate picture of whether your attachment will make it through.

You'll also find a complete reference of which file types get blocked (executables, scripts, disk images), which are always safe (PDFs, images, documents), and why marketing emails should never include attachments in the first place.

If you're building email campaigns, our email word counter helps you make sure the total email (HTML + images) stays under clipping thresholds. For signature images, check our signature size guide, and for campaign header images, see our email banner size guide. Our deliverability score tool can help you check if attachments are affecting your sender reputation.

Frequently Asked Questions

What is the maximum email attachment size for Gmail?

Gmail allows attachments up to 25MB. If your file is larger, Gmail automatically offers to upload it to Google Drive and share a link instead. Remember that Base64 encoding adds ~37% overhead, so a file over ~18MB might already exceed the 25MB limit after encoding. Google Workspace (business Gmail) has the same 25MB limit for attachments but allows larger files via Google Drive sharing.

What is the maximum email attachment size for Outlook?

Outlook.com (personal/Hotmail) allows up to 20MB. Microsoft 365 (business Exchange Online) defaults to 35MB but admins can increase it up to 150MB. However, the receiving email server also needs to accept that size — most corporate servers cap at 25-35MB. For larger files, Outlook integrates with OneDrive for link-based sharing.

Why is my attachment getting rejected even though it's under the limit?

Almost certainly because of Base64 encoding. When you attach a file to an email, it gets converted from binary to text using Base64 encoding. This process increases the file size by approximately 37%. So a 20MB file becomes ~27.4MB after encoding, which exceeds Gmail's 25MB limit. Always account for this overhead when estimating if your file will fit.

What file types are blocked by email providers?

Most providers block executable files (.exe, .bat, .cmd, .msi, .scr), script files (.js, .vbs, .ps1), and disk images (.iso, .img). Gmail is particularly strict — it scans inside ZIP files and blocks archives that contain executables, even nested ones. The safest file types are PDFs, Office documents (.docx, .xlsx, .pptx), images (.jpg, .png), and plain text (.txt, .csv).

Should I include attachments in marketing emails?

No. Attachments in marketing emails cause several problems: spam filters flag them (it's a classic spam pattern), they slow down delivery (multiply file size × thousands of recipients), and you can't track downloads. Instead, host the file on your website or cloud storage and include a download button or link. This gives you click tracking, faster delivery, and avoids spam filter issues.

How can I reduce my attachment size?

For images: compress with TinyPNG or ImageOptim, reduce resolution to 1920px max width, use JPEG at 80% quality. For documents: convert to PDF (usually smaller than DOCX/PPTX). For any file type: use ZIP compression which can reduce size 50-80%. For presentations: compress embedded images within PowerPoint before exporting. Or use a cloud storage link instead of attaching.

What is the safest way to send large files via email?

Upload to cloud storage (Google Drive, Dropbox, OneDrive, or your own server) and share a download link. This avoids size limits entirely, gives you access controls (password protection, expiry dates), provides download tracking, and doesn't trigger spam filters. Most email clients have built-in cloud integration that makes this seamless.

Do attachment limits apply per file or per email?

Per email (total). If Gmail's limit is 25MB, that's the total for all attachments combined, not per file. So you can't send five 10MB files in one email. The limit also includes the email body itself (HTML, text, headers), though that's usually negligible compared to attachment sizes. When multiple attachments approach the limit, it's better to split them across multiple emails or use cloud storage links.

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