Trim video

Keyframe Fast cut or precise re-encoding. MP4/WebM when supported.

Set the in and out points on the timeline and choose how to cut. Fast copies the compressed video and audio without re-encoding: it is almost instant and loses no quality, but a copy can only begin on a keyframe, so Nerulio starts the clip at the first keyframe inside your selection. Precise decodes and re-encodes the section, so it starts and ends exactly where you set them, at the cost of time and one more generation of compression. The result is MP4 (H.264 when your browser can encode it) or WebM.

Drop video files hereProcess many files at once

No upload · processed on this device

Making a game?

Nerulio is a 2D game asset studio first: draw pixel art, cut sprite sheets into animations, pack texture atlases, check autotile tilesets and export them for Godot, Unity, Phaser and other engines. It runs in your browser; files stay on your device.

Open the game studio →All game tools GIF to sprite sheet Sprite animation preview Sprite sheet packer

Keyframes, and why a copy cannot start anywhere

Compressed video stores a complete picture only now and then. These keyframes (I-frames) can be decoded on their own; the frames between them store only changes and need the frames before them. How far apart keyframes are depends on the encoder and its settings: a fraction of a second in some files, several seconds in others.

A cut that copies packets therefore has to start on a keyframe, and tools choose differently. FFmpeg documents that with stream copy it seeks to the closest seek point before the requested time and keeps that extra piece, so the clip starts early. Nerulio's Fast mode shrinks instead: the video starts at the first keyframe inside your selection, so nothing you excluded appears, but up to one keyframe interval at the start can be lost. The end stays at your out point.

Precise mode decodes every frame of the section and writes a new stream that begins with its own keyframe at your in point. It can also cap the resolution or remove the sound. Fast mode can remove the sound too, but it ignores the resolution cap because nothing is re-encoded.

Keyframe (I-frame): A frame stored as a complete picture; decoding can start there.
GOP: Group of pictures: a keyframe and the frames that depend on it, up to the next keyframe.
Stream copy (remux): Moving compressed packets into a new container without decoding: fast, lossless, bound to keyframes.

Example: keyframes every 2 s, selection from 3.40 s to 9.00 s

The same selection cut three ways:

Keyframes in the source        0.00  2.00  4.00  6.00  8.00  10.00 s
Your selection                 3.40 → 9.00 s   5.60 s
Nerulio Fast                   4.00 → 9.00 s   5.00 s   first 0.60 s dropped
Copy from the earlier keyframe 2.00 → 9.00 s   7.00 s   1.40 s extra at the start
  (FFmpeg -ss before -i, -c copy)
Nerulio Precise                3.40 → 9.00 s   5.60 s   re-encoded

If the first 0.60 s matter, move the in point back onto the keyframe at 2.00 s or use Precise. Nerulio's test suite accepts a fast cut within 1.1 s of the requested length and a precise cut within 0.12 s.

Fast or Precise

	Fast · keyframes	Precise · re-encode
Start point	First keyframe inside the selection	Exactly your in point
Speed	Copies packets; mostly limited by reading the file	Decodes and encodes every frame
Picture quality	Identical to the source	One more generation of compression
Codecs in the result	The source codecs	H.264 in MP4, or VP9, VP8 or AV1 in WebM
Resolution cap and mute	Mute works, the cap is ignored	Both apply
Needs a decoder	No	Yes, for the video and the audio codec

Check the result

Compare the result's length in the player with the span under the timeline: Fast may be shorter at the start, Precise should match.
Play the first second of a fast cut: it begins on a clean full frame, later than your in point when no keyframe was there.
Check that the sound is there (unless you chose Remove sound): the export stops with an error instead of silently dropping a track.

Troubleshooting

Symptom	Likely cause	How to check	Fix
A fast cut to WebM stops with "A required video/audio track cannot be preserved"	WebM accepts only VP8, VP9 or AV1 video and Opus or Vorbis audio, so H.264, HEVC or AAC from an MP4 or MOV cannot be copied into it	The source is MP4 or MOV	Choose MP4 for fast cuts, or use Precise
The fast clip starts later than the in point	There is no keyframe at the in point, so the copy starts at the next one	Compare the result length with the selection	Move the in point earlier, or use Precise
MP4 is missing from the format list	The browser reports no H.264 encoder	Only WebM is offered under Advanced	Export WebM, or use a browser with an H.264 encoder
The export stopped when you switched tabs	Only the compatibility recorder, used without WebCodecs, needs a visible tab	The compatibility note is shown	Keep the tab visible, or use a browser with WebCodecs
A fast MP4 cut from a WebM will not play in some apps	The copied VP9 or AV1 video and Opus audio now sit in an MP4 container, which not every player expects	ffprobe shows the codecs	Cut WebM sources to WebM, or use Precise for H.264

Other ways to do it

Option	When it is the better choice
FFmpeg with `-ss` before `-i` and `-c copy`	Scripted or batch cuts. Per its documentation, the copied cut keeps the part from the earlier keyframe, so it includes a little extra instead of losing it.
A desktop video editor	Several clips, transitions, or edits that must be frame-exact without re-encoding the whole section.
Compress the clip	When the goal is a smaller file rather than a shorter one.

What this page does not cover

Fast cuts start on a keyframe; there is no smart mode that re-encodes only the partial group of pictures at the start.
One continuous section per export; joining sections needs a desktop editor.
Only the primary video and audio track are kept.

Versions and sources

Checked in tests/media-browser.mjs in Chromium 153 and Firefox 155: a fast cut of 1–4 s from a 1920 × 1080 H.264/AAC MP4 keeps its audio and lands within 1.1 s of the requested length; a precise cut of 1.25–3.75 s lands within 0.12 s; in the optional large-file run, a fast cut of the last minute of a 500 MB+ one-hour file came out 60 s long, within 1 s, with audio. Engine: Mediabunny 1.58.1 with WebCodecs. The FFmpeg behaviour is quoted from its documentation.

Official documentation used

Trim video

Keyframe Fast cut or precise re-encoding. MP4/WebM when supported.

How it works

Open the source file
Choose the processing mode and settings
Run, inspect the result and download

Supported formats

MP4 / WebM / MOV → MP4 / WebM — Input decoding and output encoding depend on browser support.

What this tool does

Choose inner-keyframe remux or decoded and re-encoded interval export.

Implementation and verified quality

advanced · keyframe remux / WebCodecs precise re-encode. Quality qualification is in progress. Pro/Flagship quality has not been established.

Before you export

Fast cut shrinks to inner keyframes. Precise mode re-encodes; codec availability varies. Temporary outputs use local browser storage when available. Compatibility recording retains its short-clip limits.

Frequently asked questions

What determines the supported size?

Actual device memory, storage and codec support. Tested examples are evidence, not a guarantee for every file.

Are my files uploaded?

This tool processes the files you open on your device without uploading them. Images you explicitly attach to community posts are uploaded separately. Optional engines and models are downloaded from third parties; those requests expose connection information such as your IP address.

Will my work be saved automatically?

No. Images and editing state stay in this tab; media output can use local temporary storage. Changing language preserves your work; reloading, closing the tab, or following a link to another page can clear it.