How to Compress a Video Small Enough to Email

It depends on every server the message passes through, and the lowest one wins. These are the documented figures as of 27 September 2026.

Two things jump out. The consumer webmail services everyone assumes are the tight ones are actually the loose ones at 25 MB. The tight ones are the default desktop Outlook setting at 20 MB and old on-premises Exchange at 10 MB, which is exactly where a video sent to someone’s work address goes to die.

Because email cannot carry raw binary. Attachments are converted to base64 text, which turns every 3 bytes into 4 characters and then adds a line break every 76 characters. We measured it: 19,000,000 random bytes became 25,666,670 bytes of base64 with Unix line endings, and 26,000,004 bytes with the CRLF endings email actually uses. That is a 36.8% increase before a single header is added.

head -c 19000000 /dev/urandom > test.bin
base64 -b 76 -i test.bin -o test.txt   # 25,666,670 bytes; +1 per line for CRLF

Microsoft states this outright. Its Exchange Online documentation says messages leaving Microsoft’s datacenters are “subject to an additional 33% translation encoding increase”, and that Outlook on the web deliberately restricts sends “to 25% less than the configured setting” to leave room for it. Google’s help page gives its 25 MB figure without saying which side of encoding it counts.

So borrow Microsoft’s own rule. Take a quarter off the smallest limit on the route. A 20 MB limit becomes a 15 MB file. A 25 MB limit becomes 18.75 MB. By our measured ratio the true ceilings are 14.62 MB and 18.27 MB, so the rule leaves a sliver of margin on the first and cuts it fine on the second. If you do not know what the recipient’s server is, 15 MB is the number.

Size is bitrate multiplied by duration, which means the length of the video decides almost everything. The table shows the video bitrate you can afford once 96 kbps of AAC audio is taken out, in decimal megabytes. It is arithmetic, not a measurement, and our Discord guide shows the formula landing within 2% of real encodes.

Anything under a minute is easy at 1080p. By three minutes you are working with well under a megabit per second, and by ten minutes the numbers stop describing video anyone would want to watch. That is not a compression problem. That is a sign you should be sending a link.

Usually the resolution, but not always, and the difference is large enough to test. We gave two clips the same 780 kbps budget, roughly what a three-minute video gets at 20 MB, and encoded each at three resolutions with two-pass H.264. VMAF scores each result against the original at 1080p, on a 0 to 100 scale.

On the calm clip, 1080p won. On the busy one, 1080p scored 49 and 540p scored 89 at a smaller file size. Same budget, opposite answers. At a low bitrate the encoder has a fixed number of bits per second to spend, and a busy, noisy frame at 1080p asks for more than it can give. Fewer pixels means each one gets more.

A practical rule falls out of it. If your phone was on a tripod or the scene barely moves, keep the resolution and let the bitrate drop. If it was handheld, crowded, dark or grainy, go down to 540p. And notice that 720p lost to 540p on both clips: 960 × 540 is exactly half of 1080p in each direction, so the scaler averages clean blocks of four pixels, while 720p is a two-thirds scale that needs more interpolation.

# two-pass H.264 at 780 kbps video, 960x540, 96 kbps audio
ffmpeg -i in.mov -vf scale=960:540 -c:v libx264 -pix_fmt yuv420p -preset medium -b:v 780k \
  -pass 1 -an -f null /dev/null
ffmpeg -i in.mov -vf scale=960:540 -c:v libx264 -pix_fmt yuv420p -preset medium -b:v 780k \
  -pass 2 -c:a aac -b:a 96k -movflags +faststart out.mp4

H.264 in an MP4. Not because it is the best codec, but because you do not control what the recipient opens it on. An H.265 file is smaller at the same quality, but on Windows it needs Microsoft’s HEVC decoder, the same one that makes HEIC photos refuse to open on a PC, and the paid version of that package costs $0.99. Nobody wants to buy a codec to watch your kid’s recital.

If you know the recipient is on a recent iPhone or Mac, H.265 is fine and buys you real headroom. How much depends on the footage: in our H.265 versus H.264 measurements the saving at equal quality ranged from under 3% on a calm clip to 39% on a busy one.

Add -movflags +faststart when you export with ffmpeg. It moves the index to the front of the file so a webmail preview can start playing before the whole attachment has downloaded.

Whenever the budget table says the video would have to fall below about a megabit per second. At that point you are trading real quality for the convenience of an attachment, and there are free ways to keep both.

Mail Drop is built into Mail on the Mac and into iCloud Mail. Apple documents it at up to 5 GB per message, with links that stay live for 30 days and nothing counted against your iCloud storage, subject to a 1 TB Mail Drop allowance. The recipient gets a download link that works on any platform. For a family video, this is simply the correct answer.

Gmail does the equivalent automatically: attach something over 25 MB and it becomes a Google Drive link. Outlook.com offers OneDrive attachments up to 2 GB. In all three cases the video arrives at full quality and nobody had to think about bitrates.

The one case where compressing still wins is a recipient who will not or cannot click through to a download: an automated inbox, a strict corporate gateway, a form that only accepts attachments. For those, the 15 MB target and the resolution rule above are what you need.

For one video, QuickTime Player is free and already installed. File → Export As → 720p or 480p gets most short clips under the limit with no settings to think about. Our video compression guide measures what each export preset produces.

To hit an exact size, ffmpeg or HandBrake is more direct. Smol has no “make this 15 MB” control for video. It gives you codec, CRF, encoder preset and output width, and CRF targets quality rather than size. Two-pass ffmpeg as above, or HandBrake’s average-bitrate mode with two passes, lands on a byte budget in one go.

For long videos, Mail Drop beats every compressor, ours included, because the best-compressed ten-minute video at 20 MB is still worse than the original behind a link.

We also checked whether Smol does anything ffmpeg does not. It does not, and we would rather you heard it here. With the settings pinned on both sides, Smol set to H.264, CRF 23 and the medium preset produced a file of exactly the same size as ffmpeg’s libx264 at CRF 23 medium: 1,757,028 bytes each. Set to a width of 1280, it matched ffmpeg’s default scaler to the byte again, at 639,912. The encoder under the hood is the same libx264, and its output width choices are original, 3840, 1920, 1280 and 854, so it cannot do the 960 × 540 that won the busy-clip test above.

What the $29 buys is not a better encoder. It is not having to remember any of this: a Finder right-click, a batch of forty clips from a trip in one drop, and the same app handling the photos and PDFs that usually travel with them. If you send video by email once a year, use QuickTime and Mail Drop. If you do it every week, see how the batch tools compare and everything Smol reads and writes.