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BPM Detector

Drop in a track. Get its tempo and the exact time of every beat — computed on your own machine, with nothing uploaded.

What it measures

Tempo detection starts by finding onsets — the moments when energy rises sharply across the spectrum, which is what a drum hit, a plucked string or a syllable looks like to a machine. Those moments form a jagged curve over time. A steady pulse shows up as a repeating pattern in that curve, and the strongest period is the tempo.

Knowing the tempo is not the same as knowing where the beats fall, so a second pass places a grid: it searches for the sequence of beat positions that both lands on strong onsets and stays evenly spaced, trading one against the other. That is why the beats returned here sit on the music rather than on a metronome started at zero.

Why it runs in your browser

Every comparable tool asks you to upload the track first. That is a consequence of how they are built, not a requirement of the problem — the analysis is a few million floating-point operations, which a phone does without noticing.

This one is the montage engine from Life2Film, compiled to WebAssembly. The same code decides where to cut when the app assembles a video, and it runs on your device there for the same reason it does here: sending someone’s footage to a server to answer a question the device can answer is a cost, not a feature.

Your file is read and re-encoded by your own browser. No request carries it anywhere — this page has no upload endpoint to send it to.

Questions

Does my audio get uploaded anywhere?

No. The analysis runs inside your browser as WebAssembly. The file is read from disk into memory and never sent over the network — you can check by opening the network tab, or by pulling the plug once the page has loaded.

How accurate is it?

It uses onset-strength analysis with dynamic-programming beat tracking — the same approach behind established music-information-retrieval tooling, and the same code that drives Life2Film’s montage engine. On music with a steady pulse it lands within a beat of the truth. On rubato, live recordings without a click, or heavily syncopated material, treat the number as a starting point.

What does “half or double tempo” mean?

Tempo is ambiguous by nature: a track at 140 BPM is also, correctly, 70 BPM counted in half time. Detectors pick whichever is stronger in the signal. If the number feels off by exactly 2×, that is what happened, and both answers are shown so you can take the one you need.

Which formats work?

Whatever your browser can decode: MP3, WAV, M4A, AAC, FLAC and OGG in current Chrome, Edge, Safari and Firefox. Video files work too — the audio track is what gets read.

Can I use the beat times in my editor?

That is what the export is for. EDL is the one to pick for DaVinci Resolve or Premiere Pro — each beat arrives as a named marker on the timeline. Final Cut takes the FCPXML. OTIO works in Resolve, Avid and Premiere too, and is the right choice if the beats are heading into a pipeline rather than straight onto a timeline. Audacity reads the label export, and CSV or JSON cover spreadsheets and scripts.

Why does the frame rate matter?

Beats fall on fractions of a second; timelines count whole frames. Every marker has to be rounded to the nearest frame of the project it is going into, so a grid exported at 25 fps will sit up to 20 ms off in a 30 fps sequence. Set the rate to match your project and the markers land exactly where the beat is. It does not affect CSV or JSON, which carry raw seconds as well.

The other tools

  • Trim Video — Drag the handles to the part you want, preview it, save it. Audio stays in sync.
  • Compress Video — Make a file smaller and see exactly what it cost. No size limit, because there is no server.
  • Video to MP3 — Pull the audio out of a video and keep it as MP3, WAV or OGG.
  • Convert Video — MP4, WebM and MOV in any direction — or pull the audio out as WAV.
  • Video Splitter — Cut a long video into Reels or Shorts that start on a shot change instead of every 30 seconds, ranked by picture quality.
  • BPM Detector — Find a track’s tempo and the time of every beat, then send the grid to Resolve, Premiere, Final Cut or Audacity as markers.
  • Scene Detector — Find every cut in a video and get the shot list with thumbnails, timecodes and quality scores.

Build this into your own project

Finding the tempo and beat grid of a track — the same way this page does it, on the user's machine, with no server. Take the code, or hand the prompt to a coding agent. 

const engine = await import('https://life2film.com/wasm/va_wasm.js');
await engine.default({ module_or_path: 'https://life2film.com/wasm/va_wasm_bg.wasm' });

const ctx = new AudioContext();
const buffer = await ctx.decodeAudioData(await file.arrayBuffer());

// Down-mix to mono at 22050 Hz first (OfflineAudioContext), then:
const { bpm, beats } = JSON.parse(engine.detect_beats(mono, 22050, null));

The engine is documented for agents, and every tool page is available as markdown by adding .md to its address.