Video Merger

Source videos:

Choose 2–6 MP4, MOV, M4V, WebM, or MKV clips. Each clip may be up to 40 MiB and 90 seconds; the combined timeline may be up to 120 MiB and 3 minutes.

Output order:

The first clip sets the output frame. Later clips are contained with black letterboxing when their shape differs.

{{ clip.file.name }} {{ formatDuration(clip.duration) }} · {{ clip.width }} × {{ clip.height }} · {{ clip.hasAudio ? 'Audio' : 'Silent' }}

Sound:

Keeping sound inserts silence where needed so video and audio stay aligned.

Output format:

Choose MP4 for broad editor support or WebM for a browser-native VP8/VP9 path.

Output filename:

Neutral default: blank. A safe `.{{ outputExtension }}` extension is added automatically.

{{ row.label }}{{ row.display }}

The chart renderer is unavailable. The same join positions remain available in the clip ledger.

Order	Source	Input frame	Start	End	Duration	Sound action	Copy
{{ row.order }}	{{ row.name }}	{{ row.frame }}	{{ row.startDisplay }}	{{ row.endDisplay }}	{{ row.durationDisplay }}	{{ row.audioAction }}

Video clips rarely share every technical property just because they play on the same device. Different cameras and apps may produce different frame sizes, aspect ratios, frame rates, codecs, and audio tracks. A dependable join first gives those clips one output shape and timeline, then places them end to end.

That normalization prevents a common failure in simple concatenation, where a player reaches the next clip and encounters dimensions or media settings that no longer match. It also introduces real choices. Fitting a portrait clip inside a landscape frame creates black bars, converting to a shared frame rate changes frame timing, and re-encoding can alter image or sound quality.

Container: The file structure, such as MP4 or WebM, that holds encoded video and optional audio tracks.
Codec: The method used to encode a track, such as H.264 for MP4, VP8 or VP9 for WebM, AAC for MP4 audio, or Opus for WebM audio.
Letterboxing: Black space added when a clip is fitted inside the output frame without stretching or cropping its picture.

Sound also needs a continuous timeline. When available audio is kept, a silent segment must fill any clip that has no audio track so later speech and pictures stay aligned. Muting removes the output audio track entirely rather than leaving a sequence of silent audio.

A playable file is not the whole review. Watch every join, listen across sound transitions, and check titles or action near the frame edges. Automated duration and frame checks cannot judge a missing beat, an awkward cut, or unwanted letterboxing.

How to Use This Tool:

Choose the lead clip carefully because its frame size becomes the output frame for the complete timeline.

Select 2 to 6 MP4, MOV, M4V, WebM, or MKV clips. Each may be 0.05 to 90 seconds and no larger than 40 MiB; the set may total 120 MiB and 3 minutes.
Place the clips in final playback order. Use a first clip with the aspect ratio and dimensions you want for the result; later clips are contained inside that frame with black letterboxing when needed.
Set Sound to keep available audio or mute the merged video. Keeping sound inserts silence for clips or portions without audio so the soundtrack remains aligned.
Choose MP4 · H.264 for broad editor and player support or WebM · VP8/VP9 for a browser-oriented path. If the browser lacks the required encoder, try the other format or mute WebM where the local fallback is available.
Merge, then watch the complete output. Confirm the reported duration, output frame, sound policy, clip start and end times, and each visual and audible join before downloading.

Interpreting Results:

The timeline should contain one continuous segment per selected clip with no overlap. Total duration is the sum of inspected clip durations. Small differences in the verified file can occur because encoded frames and audio samples land on discrete time boundaries.

Preserve means a clip already matches the output frame dimensions.
Fit and letterbox means the picture is scaled to fit without cropping and unused frame area is black.
Keep, insert silence, and mute describe how each clip contributes to the output sound track.

Treat the output checksum as an identity for that generated file, not proof of visual quality or authenticity. Playback review is still required after any browser, format, codec, or ordering change.

Technical Details:

A normalized audiovisual join decodes the primary video track from each source, places its samples on a new cumulative timeline, fits every frame into one even-sized output box, and encodes a new file. Only the primary video and primary audio tracks are considered; extra tracks and source metadata are not carried into the result.

Formula Core:

The output duration adds the inspected clip durations. The first clip supplies the output width and height, with an odd final pixel removed because the encoded frame dimensions must be even.

\begin{array}{rcl} T_{output} & = & \sum_{i = 1}^{n} T_{i} \\ W_{output} & = & W_{1} - (W_{1} mod 2) \\ H_{output} & = & H_{1} - (H_{1} mod 2) \end{array}

For three clips lasting 12.4, 8.1, and 5.5 seconds, the planned duration is 26.0 seconds. A first clip measuring 1,281 × 721 pixels produces a 1,280 × 720 output frame; later clips are fitted inside that box.

Transformation Core:

Video merge normalization and encoding stages
Stage	Transformation	Output rule
Inspect	Read an ISO media or EBML signature, primary tracks, duration, display width, and display height.	Each clip must be decodable, 1080p or smaller, and within file and duration limits.
Plan	Use the first clip's even dimensions, selected container, and sound policy.	Video becomes 30 fps with contain fitting and black letterboxing.
Place video	Decode frames, fit them to the output box without alpha, and shift timestamps to the cumulative start of each clip.	Each clip begins when the previous inspected duration ends.
Place audio	Keep primary audio at 48,000 Hz stereo, fill internal or clip-wide gaps with silence, or omit the track when muted.	MP4 uses AAC; WebM uses Opus when sound is kept.
Encode	Write H.264 video in MP4 or VP9 video in WebM, with a muted WebM recording fallback that may use VP8 or VP9.	Actual availability depends on browser encoders.
Verify	Open the generated file again and check the video track, dimensions, duration tolerance, and sound-track policy.	Duration tolerance is the larger of 0.2 seconds or 8% of the planned total.

MP4 creation timestamps are cleared before the file checksum is calculated. The checksum identifies the final bytes, while source filenames are used only for the clip ledger and output review.

Compatibility and Privacy Notes:

Source clips are inspected, decoded, normalized, encoded, verified, and previewed in the browser; they are not uploaded for video processing. The browser may need a network connection to load the media engine before local work begins.

A listed filename extension does not guarantee that the browser can decode the file's actual codec.
Encoder support varies by browser, operating system, and hardware. MP4 with H.264 is not available everywhere.
Re-encoding can change image and sound quality, frame cadence, metadata, and file size.
Keep the source clips until the complete merged file has been reviewed in the destination player or editor.

References:

FFmpeg concat filter documentation, FFmpeg.
Converting media files, Mediabunny.