Song BPM Finder (BPM Detector)

Find the tempo of a song in beats per minute from the audio file itself: for DJ mixing, choosing workout or running music, setting a metronome to practise along, or syncing a video edit. The finder listens to up to two minutes, measures when new sounds start, and looks for the beat period that repeats most strongly. You get the BPM to one decimal, the half- and double-time values and a confidence rating. The song never leaves your browser.

How to use this tool

1. Choose one or more songs.
2. If the song has a slow or beatless intro, set a start time in the main part.
3. Pick the range the result should fall in (80–160 BPM suits most music).
4. Press Calculate and read the BPM, alternatives and confidence.

Formula

BPM = 60 × frame rate ÷ beat period (frames) · frame rate = 11,025 ÷ 256 ≈ 43.07 per second

The audio is decoded at 11,025 Hz in mono and analysed in 1,024-sample windows every 256 samples. For each step the onset strength is the sum of increases of the log spectrum (spectral flux); a 1-second moving average is subtracted. The autocorrelation of that curve peaks at the beat period and its multiples, so every tempo from 40 to 240 BPM is scored by the average autocorrelation at 1, 2, 3 and 4 beat periods, and the best one inside the chosen range wins. In the wide 60–200 range, the best tempo and its double or half (when at least half as strong) are compared, and the one nearer 120 BPM is reported. The period is then refined from the peaks at its first eight multiples. Beat strength is that average (0 to 1); below 0.1 there is no clear beat, below 0.2 the confidence is low.

Example

A house track whose kick drum hits every 0.469 s has a beat period of 0.469 × 43.07 ≈ 20.2 frames, so the BPM is 60 ÷ 0.469 = 128.0. Half time is 64 BPM and double time 256 BPM; in the 80–160 range, 128 is reported.