BPM Detector

What is a BPM detector?

A BPM detector analyzes a track and estimates its tempo in beats per minute, along with its musical key, without you needing to count beats by hand or already know the song. This one reports a confidence score with every tempo estimate, offers half-time and double-time alternatives when the reading could reasonably go either way, and includes a tap-tempo pad so you can confirm or set the tempo yourself by tapping along.

Alongside tempo, it detects the track's key and mode, converts that into a Camelot code for harmonic mixing, and reports basic technical details — duration, sample rate, and integrated loudness in LUFS — all from a single pass over the file, all processed on your device.

How tempo detection works, in plain words

Automatic tempo detectors generally combine two ideas. The first is onset detection: scanning the audio for moments where energy suddenly increases, which usually corresponds to a drum hit, a plucked note, or any other percussive attack. That produces a rough map of when things happen in the track.

The second is autocorrelation: comparing that pattern of onsets against a time-shifted copy of itself to find the spacing that repeats most consistently. The spacing that lines up best, across the whole track rather than just one section, becomes the tempo estimate. A track with a strong, steady beat produces a clear, repeating pattern and a high confidence score; a track with a loose, sparse, or constantly shifting rhythm produces a weaker pattern and a lower one.

Half-time and double-time: reading the alternatives

Tempo can be genuinely ambiguous, not just hard to measure. A beat you could tap on every pulse is just as easy to tap on every other pulse instead, and both are defensible readings of the same rhythm — this is especially common in electronic, hip-hop, and dance music where the underlying pulse can be felt in more than one way. When that's the case, the detector shows a primary estimate plus its half-time and double-time alternatives, so you can pick whichever matches how you actually count the track.

If the confidence score is low or a result looks off, the tap-tempo pad is the fastest way to settle it: tap along to the beat yourself for a few bars, and that overrides the automatic reading with something grounded in how you hear the track.

Key, mode, and the Camelot wheel

Alongside tempo, the detector reports the track's musical key, its mode (major or minor), and a Camelot code — a two-character label combining a number from 1 to 12 with a letter, A for minor keys and B for major keys, used widely for harmonic mixing. Camelot codes make it easy to see at a glance which keys are musically close to each other without needing to think in sharps, flats, and modes directly.

The relative key is also shown: every major key has a relative minor that shares the same key signature, and the Camelot wheel places them on the same number, one on the A side and one on the B side. The full mapping is below.

Harmonic mixing basics

Camelot codes exist mainly to make key compatibility easy to judge without music theory. As a starting point, tracks that share the same code are in the same key and mode, so they generally layer or transition cleanly. Moving to an adjacent number with the same letter, say 8B to 9B, shifts by a perfect fifth, one of the smoothest possible key changes and a common way DJs build a set that gradually moves through related keys.

Switching the letter while keeping the number, like 8B to 8A, moves between relative major and minor — the same key signature, different tonal center — which is another common, low-friction transition. None of this guarantees two tracks will sound good together; tempo, energy, arrangement, and mix all matter too. It's a starting point for narrowing options, not a substitute for listening.

Batch analysis and CSV export

When you need results for more than one file, batch analyze processes multiple tracks in sequence and collects tempo, key, Camelot code, duration, sample rate, and loudness for each one into a single table, instead of making you open and read files individually. It's built for going through a folder of tracks at once rather than one-off lookups.

From that table, download a CSV file with every result. A CSV opens directly in a spreadsheet or imports into most DJ library and playlist tools, which makes it a practical way to tag or sort a batch of tracks by tempo or key without retyping anything by hand.

Honest limits of automatic detection

Tempo and key detection are estimates, not certainties, and it's worth knowing where they're weaker. A track whose tempo changes partway through, common in DJ mixes, live recordings, or songs with a deliberate tempo ramp, will produce one estimate for a signal that doesn't actually have a single fixed BPM — the confidence score tends to drop in these cases, which is a useful signal on its own.

Key detection can also be less certain on tracks that modulate to a different key partway through, lean on ambiguous or modal harmony, or don't have much tonal content to begin with, like a mostly percussive drum loop. Treat both tempo and key results as strong starting points to confirm by ear, especially when the confidence score is low or the half/double-time alternative looks just as plausible as the primary reading.

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