Should I trim the video or compress it first?

Trim first, every time. Cutting a clip in half removes half the bytes and costs nothing in picture quality, because the default trim is a stream copy rather than a re-encode. Compressing harder buys you a similar saving but pays for it in smearing around motion and text.

How small does the file actually need to be?

Most mail systems stop somewhere between 10 MB and 25 MB, and your recipient's server has its own limit that may be lower than yours. Attachments are also encoded for transport, which adds about a third to the size on the wire, so aim comfortably under whatever number you have been given rather than right at it.

Why is there a one-time download before I can compress?

Compression runs a full ffmpeg build compiled to WebAssembly, about 31 MB, fetched on first use and then cached. The strip above the drop zone shows the size and offers a Save engine for offline button if you would rather pull it in advance. That download is the program, not your video.

Does the video get uploaded to a server?

No. Reading the file, building the thumbnail timeline, and the encode itself all happen in the browser tab. The only network request involved is fetching the engine, and that is the same file for everyone.

Would sending a GIF be smaller than an MP4?

Almost never, beyond a few seconds. GIF is limited to 256 colours per frame and uses very old compression, so a ten-second GIF often ends up larger than the compressed MP4 it came from. Video to GIF offers an animated WebP option instead, which keeps full colour and is usually far smaller.

Compress a Video for Email Without Uploading It

Get a video under your mail server's attachment limit: trim the clip first, then compress it to 720p in your browser. Nothing uploads, and there's no watermark.