Compress Video for Email: Real Size Limit

Aim for a video file of about 18MB, not 25MB. Gmail and most other providers cap a message at 25MB, but the cap counts the whole encoded message, and email turns every attachment into text that is roughly 35% larger than the file itself. A 25MB video is rejected even though it looks like it fits. To compress for email, open the [Compress Video tool](/compress-video), choose By target size, enter 18MB as a custom size, and download the result. The clip is re-encoded in your browser, so the footage never leaves your device. What matters most is length: a one-minute clip fits at good quality, three minutes fits at acceptable quality, and ten minutes does not fit without looking badly degraded.

Email can only carry text, so an attachment is converted to base64, a text form that uses four characters for every three bytes, and it is split into short lines that add a little more. We measured it by attaching an 18,000,000-byte video to a message with Python's standard email library: the finished message was 24.3MB, a 1.35 times increase. Divide 25MB by 1.35 and you get about 18.5MB, which is the largest attachment that fits, before the body text and headers. Rounding down to 18MB leaves a little room. Outlook.com caps attachments at 20MB, so for an Outlook recipient the same arithmetic gives about 14MB.

File size is bitrate times duration, so the longer the clip, the thinner each second gets. Running the Compress Video planner with an 18MB target gives these video bitrates, with audio at 128 kbps unless noted: 30 seconds gets about 4,300 kbps, which is sharp HD. One minute gets 2,080 kbps, still good at 720p. Two minutes gets 976 kbps, fine for a talking head. Three minutes gets 608 kbps, soft but watchable. Five minutes gets 314 kbps, visibly blocky in motion. Ten minutes gets 125 kbps with the audio cut to 96 kbps, which is poor. The tool never goes below 100 kbps of video, so a 15-minute clip would still come out larger than 18MB. See [what bitrate means](/blog/what-is-bitrate) for why.

Open the Compress Video tool and drop in your file. Switch the mode to By target size and enter 18 as a custom size in MB. The tool reads the clip length, works out the bitrate from the table above, and shows the plan before it starts. Choose a resolution cap of 720p if the clip is longer than two minutes, because fewer pixels at the same bitrate look cleaner than many pixels starved of data. Start the compression and wait; it runs inside your browser, so speed depends on your computer. Download the MP4 and attach it. If the result is still too large, the clip is too long for the limit, and trimming it with the [Trim Video tool](/trim-video) first is the quickest fix.

The tool has a one-click Email (25MB) preset next to Discord and WhatsApp. It aims the encoded video at 25MB exactly as written, so the attachment lands near the limit before email's text encoding adds its 35%. For a Gmail send, type 18 as a custom size instead of using the preset. The preset still makes sense when the 25MB figure is a plain file-size limit, as on an upload form that takes the raw file. The same reasoning applies to any limit that applies to the whole message rather than the file, which is worth checking before you trust a round number.

If the clip is over about five minutes and you need it to stay sharp, compression cannot solve it, because the arithmetic does not allow it. Better options are to trim the clip to the part the recipient needs, split it into two emails, or send a link instead. Gmail itself does this automatically: attachments past its limit are offered as a Google Drive link. A link keeps the original quality and avoids a second round of compression. If the recipient must receive a file, see [why a video sometimes will not shrink](/blog/video-wont-compress-smaller) for the cases where the tool hits its floor.

Compress once, from the original recording, because every re-encode of an already compressed file loses a little more detail. Screen recordings and slideshows compress far better than footage with constant camera motion, so a long screen capture may fit where a short action clip would not. Dropping the resolution is gentler than dropping the bitrate: 720p at 1,000 kbps looks better than 1080p at the same rate. For the same method applied to a different limit, the [10MB guide](/blog/compress-video-to-10mb) has the full bitrate maths, and the [general compression guide](/blog/compress-video-online) covers the quality slider.