How to Compress a File for an Email Attachment Limit
Most email providers cap attachments at 20-25MB, and the bounce-back error rarely tells you the file was too big. Here's how to get under the limit without losing quality.
You attach a file, hit send, and get a bounce-back — or worse, silence, and you only find out days later that it never arrived. Attachment size limits are one of the most common, least-explained reasons an email fails to deliver.
The typical limits
- Gmail: 25MB per email (attachments over this get auto-converted to a Google Drive link instead of failing outright, which can itself look unprofessional or get blocked by strict recipients).
- Outlook/Microsoft 365: 20MB in many configurations, though this varies by organization's IT policy.
- Many corporate mail servers: often capped lower than the consumer default — 10MB is common — with no advance warning to the sender.
Since you usually don't know the recipient's exact limit, the safe move is compressing well under the smallest common threshold rather than assuming the largest one applies.
What actually inflates attachment size
- Scanned documents saved as high-DPI images — a phone-scanned PDF at unnecessarily high resolution routinely hits 10-20MB for content that would be perfectly legible at a fraction of that size.
- Multiple photos bundled into one PDF without any compression pass.
- PNG used for photographic content — PNG is lossless and compresses photos far worse than JPEG for the same visual quality.
The fix
- If it's a PDF, compress it to a specific target — well under 25MB, ideally under 5-10MB if you want real headroom for stricter recipient servers.
- If you're attaching several photos, consider whether they need to be full camera resolution — compressing each one before attaching (or before combining into a PDF) usually cuts the total dramatically with no visible quality loss at normal viewing size.
- If the total is still too large even compressed, that's usually a sign to use a file-sharing link instead of an attachment — but for anything sensitive, check whether the recipient's organization allows external links before defaulting to that.
A practical target
Rather than compressing to just-under-25MB and hoping, aim for under 5MB for anything you're not certain the recipient's mail server can handle. It's rarely noticeably worse in quality, and it sidesteps the entire class of "why didn't my email arrive" problems caused by attachment limits you can't see from your side.