Why 18 MB, when the limit says 25?
Outlook and most other email services cap attachments at roughly 20 to 25 MB, and Gmail allows 25 MB per message, for all attachments together, so two 15 MB videos won't go in one email. That number is a ceiling, though, not a target.
Attachments are encoded as text for the journey, which makes them about a third bigger in transit, and some services count that larger size against the limit. The recipient's provider may also be stricter than yours, and company mail servers can be stricter still. Starting at 18 MB keeps you clear of all three. If a message still bounces, compress again at 15 MB.
How much video fits in 18 MB?
The budget is spread across the whole video, so length decides how sharp it can be. Keeping the sound at 128 kbps, the picture gets roughly:
- 1 minute, about 2.3 Mbps. Sharp at 720p, fine at 1080p for calm footage.
- 2 minutes, about 1.1 Mbps. 720p for talking heads and screen recordings.
- 3 minutes, about 0.7 Mbps. 480p to 720p, best for slides and speech.
- 5 minutes, about 0.35 Mbps. 480p, ideally with the audio lowered to 64 kbps.
Past five minutes or so, an attachment stops making sense, the last section covers what to do instead.
Make sure it plays for whoever opens it
You can't know what the recipient will watch on, so the default here is deliberately the most compatible option: H.264 video in an MP4. It plays in mail-app previews, on Windows and Mac computers and on practically every phone, with no extra software.
That also fixes a common iPhone headache. iPhones record H.265 MOV files, which some Windows PCs can't play without an add-on; compressing turns them into H.264 MP4s, as long as your browser can read H.265 (Safari can, and newer machines usually manage it in Chrome or Edge). For email, leave the H.265, AV1 and WebM choices under Format and codec alone.
When it still won't fit
- Trim it. Most emailed videos have a slow start and a long goodbye. The video trimmer cuts both.
- Lower the audio. For speech, Lower (64 kbps) sounds fine and hands more of the budget to the picture.
- Halve the frame rate. Phone clips shot at 60 fps get a Frame rate option; 30 fps looks just as natural in an email and leaves each frame twice the data.
- Split it. Two shorter clips in two emails beat one unwatchable one.
- Send a link. For long videos, upload the full-quality file to a cloud drive and share the link; many mail apps offer to do this for large attachments.
To plan sizes before you start, the bitrate calculator turns a length and a target size into a bitrate. Need a different format rather than a smaller file? Try the video converter.