Email attachment limits have barely changed in 20 years. Gmail allows 25 MB. Outlook allows 20 MB. Yahoo Mail allows 25 MB. Corporate email systems often cap at 10 MB. Meanwhile, a 30-second 1080p video from a modern phone is 50-100 MB.
The math does not work without compression.
How video compression reduces file size
Video compression works by removing information the viewer is unlikely to notice. The encoder analyzes each frame and discards data in areas where the human eye has low sensitivity: subtle color variations in dark regions, high-frequency detail in fast motion, and fine texture in peripheral areas.
The Constant Rate Factor (CRF) controls how aggressively the encoder compresses. Lower CRF values preserve more detail (larger files). Higher CRF values discard more detail (smaller files).
- CRF 18: Visually lossless. You cannot see the difference from the original. File size reduction: 30-50%.
- CRF 23: Default. Very good quality. Subtle loss in fine detail that is invisible at normal viewing distance. File size reduction: 50-70%.
- CRF 28: Good quality. Some softening visible on close inspection. Fine for social media and email. File size reduction: 70-85%.
- CRF 32: Noticeable quality reduction. Visible blockiness in motion and gradients. Acceptable for quick previews. File size reduction: 85-95%.
Resolution reduction as a force multiplier
Compression alone may not get a large file under 25 MB. Combining compression with resolution reduction is more effective. A 4K video (3840x2160) downscaled to 720p (1280x720) reduces the pixel count by 9x, which dramatically reduces the file size even at high quality settings.
For email, 720p is more than sufficient. The recipient is viewing on a phone screen or a small embedded player. The difference between 720p and 1080p is invisible at email viewing sizes.
The compression workflow for email
1. Check the original file size. If it is under 25 MB, no compression needed. 2. Try CRF 28 at the original resolution first. This often reduces a 100 MB file to 15-25 MB. 3. If still too large, downscale to 720p and compress at CRF 28. This typically reduces a 100 MB file to 5-15 MB. 4. If still too large, increase CRF to 32 or reduce resolution further to 480p.
When to use a link instead
If your video is longer than 2-3 minutes, compression may not reduce it enough for email without significant quality loss. At that point, upload the full-quality video to Google Drive, Dropbox, or WeTransfer and send a download link. The recipient gets the full quality, and you avoid the attachment limit entirely.
Use email attachments for quick clips (under 60 seconds) that benefit from being directly in the email. For these, upload your clip to the MediaBrew compress tool, select CRF 28 or downscale to 720p, and attach the resulting lightweight file. Processing runs locally via FFmpeg WebAssembly so confidential attachments never upload to external servers.