# Audio Cutter & Merger

> Trim audio, keep or remove ranges, fade in/out, or join up to 10 clips with crossfades. Cuts an audio file to a start and end time, keeps or removes a list of time ranges, and adds fade-in/fade-out. In merge mode it joins up to 10 clips in order, optionally with a crossfade between them. Output as MP3, M4A, Ogg, Opus, FLAC or WAV.

Part of usefulweb.site (https://usefulweb.site/). Results are deterministic calculations from the inputs and the listed assumptions; the API returns exactly what the web page shows.

## Use it
- Human interface: https://audiocuttermerger.usefulweb.site/ (interactive tool at https://audiocuttermerger.usefulweb.site/tool)
- JSON API: POST https://audiocuttermerger.usefulweb.site/api/v1/calculate with a JSON object of inputs (schema: https://audiocuttermerger.usefulweb.site/api/v1/tool)
- OpenAPI: https://audiocuttermerger.usefulweb.site/openapi.json
- MCP (Streamable HTTP): https://audiocuttermerger.usefulweb.site/mcp - tool `audio_cutter_merger`

## Inputs
- mode (select): What to do
- file (file, required): Audio file (clip 1) - MP3, WAV, FLAC, Ogg/Opus, M4A, AIFF; max 100 MB in total. Processed on our server in a private temporary folder that is deleted afterwards; the result can be downloaded for about 2 hours.
- file2 (file): Clip 2 (merge mode)
- file3 (file): Clip 3 (merge mode)
- file4 (file): Clip 4 (merge mode)
- file5 (file): Clip 5 (merge mode)
- file6 (file): Clip 6 (merge mode)
- file7 (file): Clip 7 (merge mode)
- file8 (file): Clip 8 (merge mode)
- file9 (file): Clip 9 (merge mode)
- file10 (file): Clip 10 (merge mode)
- start (text): Start time (cut mode) - Seconds (5.5), mm:ss (1:30) or hh:mm:ss.mmm. Empty = from the beginning.
- end (text): End time (cut mode) - Empty = to the end.
- ranges (list): Time ranges (cut mode, optional) - One range per line, start-end. Overrides start/end.
- rangeAction (select): Ranges are
- fadeIn (number): Fade in
- fadeOut (number): Fade out
- crossfade (number): Crossfade between clips (merge mode)
- format (select): Output format
- bitrate (select): Bitrate (lossy formats)

## Method
- Times are parsed strictly (seconds, mm:ss or hh:mm:ss.mmm); only the resulting numbers are written into the ffmpeg filtergraph, never your text.
- Cut mode: ranges to keep are sorted, clipped to the file length and merged where they overlap; "remove" ranges are inverted into the parts to keep. Each part is cut sample-accurately with atrim and the parts are joined with the concat filter.
- Merge mode: every clip is converted to a common sample rate (the highest of the clips, max 48 kHz) and channel layout (stereo if any clip is stereo), then joined with concat, or chained with acrossfade (triangular curves) when a crossfade is set. Each crossfade shortens the total by its length.
- Fade in/out (afade) is applied to the final result. The result is encoded once in the chosen format.