How to detect the BPM
- Drop a song on the box. MP3, M4A, WAV, OGG, FLAC, WebM and the sound of MP4 or MOV video can be read.
- Leave Analyse the first at 0 to use the whole track, or enter a number of seconds (in steps of 10) to analyse only the start. That is faster on long files and useful when the intro has a different tempo.
- Press Detect BPM.
- Read Tempo, Half-time, Double-time and Beat clarity. The table Other periodicities found lists up to five other candidates with their relative strength.
What it does and when to use it
DJs use the tempo to choose tracks that mix well together. Producers need it to set a project tempo before adding loops. Video editors cut to the beat. Runners and dance teachers pick music at a target pace. Musicians learning a song set their metronome to it.
Once you know the tempo, the audio loop maker can cut a bar of exactly 60 ÷ BPM × 4 seconds. The audio speed changer can bring a song to a new tempo while keeping its pitch: a 120 BPM track at 1.05× becomes 126 BPM. To see the kick drum’s frequency, open the track in the spectrum analyser.
How it works
The track is decoded in a Web Worker on your device and mixed to mono. Two low-pass filters keep only content below about 150 Hz, where kick drums and bass sit. The tool measures energy 200 times a second, takes the rise in log-energy from one step to the next as “onsets”, and subtracts a 0.5-second moving average so long notes do not dominate. It then tests every beat spacing between 50 and 220 BPM (autocorrelation). Among the strongest results it prefers the fastest steady pulse, refines it between steps, and folds it into the 70–180 BPM range by doubling or halving.
Worked examples
From the tool’s output on synthetic kick-drum tracks:
| Test track | Tempo | Half / double | Beat clarity |
|---|---|---|---|
| 30 s at 128 BPM | 127.9 BPM | 63.9 / 255.7 | 89% |
| 30 s at 90 BPM | 90.1 BPM | 45 / 180.2 | 82% |
| Same 128 BPM track, first 10 s only | 127.8 BPM | 63.9 / 255.7 | 87% |
For the 128 BPM track the periodicity table also showed 64.2 BPM at relative strength 0.76, the half-time pulse. A 3-second clip returned “At least 5 seconds of music are needed for a tempo estimate.”
Limits and tips
- Tempo is shown to one decimal place. Round it when software asks for a whole number (127.9 → 128).
- Swing, tempo changes, live drumming and songs without bass drum can mislead it. Analyse a steady section.
- Very slow songs (under 70 BPM) are reported doubled; check the Half-time figure.
- Formats other than WAV need WebCodecs in your browser. There is no ffmpeg fallback here, so convert AIFF or WMA first with audio to WAV.
Frequently asked questions
Why does it say 128 when I count 64?
What does Beat clarity mean?
Why did it find no beat?
Does it read the BPM tag in the file?
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