Trim Video

Two trim modes, and what each one really does

This page runs FFmpeg compiled to WebAssembly inside the tab you already have open. Everything happens in your browser and nothing is uploaded to a server. On your first press of Trim video, an engine core of roughly 32 MB is fetched from a public CDN and cached, so later runs begin almost straight away.

The default setting, Fast trim, keep original quality (no re-encode), runs the equivalent of -ss START -i file -t DURATION -c copy -avoid_negative_ts make_zero. Nothing is decoded and nothing is compressed again, so picture and sound come out identical to the source and the job finishes in seconds. The catch is that a copied stream can only begin on a keyframe. If the nearest keyframe sits 1.4 seconds before the start you typed, the clip starts there instead, and some players show a brief frozen or black patch at the top.

The second setting, Precise trim, re-encode (slower, exact cut points), rebuilds the video with libx264 at CRF 23 on the veryfast preset and re-encodes audio to AAC at 192 kbps. You get a cut on the frame you asked for, at the cost of processing time and a small amount of generation loss. There is also a quiet safety net: if a stream copy completes but writes an empty file, which happens with some awkward MKV and AVI sources, the tool retries automatically in re-encode mode and says so in the log panel.

A worked example

Suppose you have stream-vod.mp4, 1.2 GB and 48 minutes long, and the moment you want begins at 22 minutes 11 seconds. Scrub the preview to that point and press Use current time beside Start, which fills in 1331.0. Scrub to the end of the moment and press Use current time beside End, giving 1352.0. Leave Fast trim selected and click Trim video. The log reads "Trimming from 1331s for 21.000s (no re-encode)" and the finished 21 second clip is saved as stream-vod-trimmed.mp4. Output always keeps the source container, so a MOV goes in and a MOV comes out.

Jobs this actually gets used for

• Pulling a 30 second highlight out of a three hour Twitch or YouTube VOD before posting it to Discord.
• Cutting the dead air off the front and back of a Zoom or Teams recording before sending it to a client.
• Shortening a phone clip so it fits under Instagram Reels or TikTok length rules without an app.
• Trimming dashcam footage down to the few seconds an insurer asked for, without handing the rest of the drive to a cloud service.

Limits worth knowing before you start

Browser tabs have a memory ceiling, and the whole file has to fit in the virtual filesystem alongside the output. Anything over 500 MB triggers an on screen warning, and very large files can simply run out of memory and stop. If that happens the tool says so plainly rather than hanging. A desktop browser handles far bigger files than a phone. Accepted extensions cover MP4, MOV, MKV, M4V plus WEBM and AVI, and anything outside that list is turned away before processing starts.

Other things this tool deliberately does not do: it will not join two clips together, crop or rotate the frame, add captions or music, change resolution or bitrate, or remove a watermark. It cuts one continuous range out of one file. End time must be later than start time, and both are entered in seconds with one decimal place rather than as a timecode. Because processing is local, the speed depends entirely on the machine you are sitting at, so a long re-encode on an older laptop will take a while.

When another tool fits better

• Need the file smaller rather than shorter? Use Compress Video.
• Want a looping animation instead of a video clip? Try Video to GIF.
• Only after the audio? MP4 to MP3 strips the soundtrack out, and Audio Converter changes the format afterwards.
• Adding burned in subtitles is a separate job for Caption Studio.
• For a still frame turned into cover art, see YouTube Thumbnail Resize.

Trimming costs nothing. Once the credit on your device is used up, a brief rewarded ad plays first, and there is never a signup, a watermark or an email wall.

Questions people ask

Is my video uploaded anywhere?

No. Trimming runs locally in the browser tab using FFmpeg compiled to WebAssembly, so the file stays on your machine. The only network traffic is the one off download of the engine itself.

Why did my fast trim start a second early?

Stream copy cannot split a compressed frame, so it snaps the start back to the previous keyframe. Keyframes are often one to ten seconds apart. Choose the precise re-encode mode if you need the cut to land on an exact frame.

What is the biggest file I can trim?

There is no hard cap, but the tab has to hold both the input and the output in memory. Files above about 500 MB get a warning, and a desktop browser copes much better than a phone.

What format will my clip come back as?

The same container you put in. An MP4 source produces an MP4, a WEBM source produces a WEBM, and the file is named after the original with "-trimmed" added.

Can I remove a section from the middle instead?

Not in one pass. This tool keeps one continuous range. To drop a middle section you would trim the part before and the part after separately, then join them in a video editor.