An MP4 can be cut online and downloaded as a trimmed WebM clip without the source ever leaving your browser tab, by using Video Trimmer to enter start and end times in seconds and select Trim video. The tool reads the chosen file locally, asks the browser to decode it, seeks the media element to the requested start point, captures the playback stream until the requested end, and writes a new WebM container from that captured stream. No file transfer to a backend, no account, and no installer are involved; everything happens inside the tab you already have open. Start and end values are decimal seconds with optional milliseconds, the resulting clip must be at least 0.1 seconds long, and the output is a finite-duration WebM file using whichever VP9, VP8, and Opus combination the browser's MediaRecorder implementation supports. Because trimming is a real-time re-encode that records the media stream while the source plays back, the encoded boundary can drift a small fraction of a second past the requested value rather than landing frame-perfect.
What Video Trimmer Takes In and Puts Out
Before opening the tool, it helps to know the boundary between what works and what the browser simply cannot decode. Video Trimmer accepts five container types: MP4, WebM, MOV, M4V, and Ogg. The accepted file can be up to 500 MiB on disk, but the decoded media must be no longer than five minutes, no wider or taller than 4096 pixels, and no greater than 3840 × 2160 pixels in total area. A familiar filename extension is not a guarantee that the file will load; the current browser must actually support the audio and video codecs inside the container. An MP4 holding H.264 video and AAC audio will typically work in Chromium-based browsers, Firefox, and Safari, while an MP4 carrying a less common codec may stall at the duration-population step.
The output is always WebM rather than MP4. Video Trimmer asks the browser's MediaRecorder to pick the first supported configuration from its VP9, VP8, and Opus list, captures the media stream produced by playing the source, and writes the recording through a shared audited Matroska writer that patches the Segment Info so compatible players report the selected duration. Dimensions are preserved from the source, but quality, keyframe placement, color metadata, audio layout, and final file size may differ because the clip is re-encoded in real time rather than copied packet-by-packet from the original container.
| Property | Limit or behavior |
|---|---|
| Accepted containers | MP4, WebM, MOV, M4V, Ogg |
| Source file size | Up to 500 MiB |
| Decoded duration | 5 minutes or less |
| Width or height | Up to 4096 px |
| Total pixel area | Up to 3840 × 2160 |
| Output container | WebM (VP9/VP8 + Opus) |
| Output accuracy | Close to the requested duration, not frame-accurate |
| Processing location | Current browser tab, no upload |
How to Cut an MP4 With Video Trimmer
The actual cut takes three short actions once the file is loaded:
- Open Video Trimmer and choose one supported local video. Wait until the duration field populates with the decoded length of the file; if it stays blank, the browser cannot decode the codecs inside the container.
- Enter the start and end times in moments. Milliseconds are written as a decimal, for example 12.5 or 47.25. The start must be zero or greater, the end must be later than the start, both must fit inside the populated duration, and the resulting clip must be at least 0.1 seconds long.
- Select Trim video, watch the reported clip duration and output size in the result panel, then download the WebM file through the link provided. If the work needs to stop mid-encode, Cancel halts the active recording.
For instance, if the populated duration is 60 seconds and you enter a start of 12.50 and an end of 47.25, the requested clip length is 47.25 − 12.50 = 34.75 seconds, which satisfies the 0.1-second minimum and fits inside the source. The recorded WebM will carry a duration close to that figure, with the final encoded frame potentially landing a small fraction of a second past the requested end because MediaRecorder samples playback time rather than exposing frame-accurate edits.
Why the Output Is WebM Instead of MP4
Video Trimmer avoids shipping a large media-encoding dependency and avoids server-side processing by relying on the codecs the browser already exposes through MediaRecorder and the Recording a media element workflow. As of current browser implementations, those codecs are VP9, VP8, and Opus packaged in a WebM container. MP4 is not produced by MediaRecorder, and adding an MP4 muxer would require either a multi-megabyte JavaScript encoder or a backend service — both of which would defeat the local, no-upload design.
If you specifically need an MP4 container for an upload pipeline, the WebM produced here can be transcoded in a separate step using a desktop tool, a command-line encoder, or a different browser-based utility that wraps a larger media stack. For most sharing use cases — messaging apps, social posts, embedding on a webpage, attaching to an email — the WebM plays directly in modern browsers and can be uploaded to platforms that accept either format.
Limits That Decide Whether Your MP4 Will Trim
Three constraints commonly decide the outcome before the user ever clicks Trim video. First, codec support inside the MP4: the browser must be able to decode both the video track and the audio track to populate the duration field. Second, decoded length: an MP4 longer than five minutes will be rejected after decoding, even if the file is well under 500 MiB on disk. Third, decoded dimensions: a 4K MP4 at 3840 × 2160 fits the 4096-pixel-per-side cap exactly, while a vertical 2160 × 3840 clip is also accepted; anything wider or taller than 4096 in either dimension fails.
Reversed or out-of-range times fail visibly rather than being silently clamped. Entering 30 as the start and 10 as the end, or asking for an end that exceeds the populated duration, surfaces an inline error so the cut does not silently become a shorter clip than the one you intended. This is intentional: a guessed trim can quietly drop the section you actually wanted to keep, while a visible rejection lets you correct the input.
| Scenario | Where Video Trimmer fits |
|---|---|
| Cutting a 90-second clip from a 4-minute MP4 | Local browser, no upload, WebM output |
| Extracting one scene from a 12-minute MP4 | Source exceeds the 5-minute decoded cap, use a desktop editor instead |
| Shrinking the trimmed WebM further | Hand the WebM to Video Compressor for a second local pass |
| Capturing a still image at a specific time | Use Video Frame Extractor for PNG export |
| Needing a frame-accurate cut for broadcast | MediaRecorder does not expose frame-accurate edits, use a dedicated NLE |
When to Use a Desktop Editor Instead
Browser-side trimming is built for speed and privacy, not for every workflow. Reach for a desktop non-linear editor when the cut needs to be frame-accurate, when the source footage runs longer than five minutes, when subtitle tracks or multitrack audio must be preserved, when exact keyframe boundaries are required for broadcast delivery, or when the downstream consumer needs the result to remain inside an MP4 container with a specific codec. Video Trimmer explicitly does not bypass DRM, does not fetch remote media, and does not promise professional frame accuracy, so any of those requirements point away from this tool.
If your goal is simply to cut an MP4 online into a shorter WebM clip — for a message, a quick share, an upload that accepts WebM, or a draft that you intend to re-edit later — Video Trimmer covers the task without sending the source anywhere. For related local tasks that often follow a cut, the general local-cutting walkthrough explains the same no-upload approach for other formats, and the Video Compressor can shrink the resulting WebM if the trimmed file is still larger than you need.
If you're weighing options, Extract Frames from Video: Test the Online Method Locally covers this in detail.