BPM, tempo and key detector for audio files

How the detector finds the tempo

When you choose a file, your browser decodes it and the page takes up to 75 seconds from just after the intro, where the beat has usually settled. It splits that into 5-millisecond slices and looks for sudden jumps in loudness, the moments where a kick, snare or strong chord hits. Kicks get extra weight because the audio is also passed through a low filter at 180 Hz, which isolates the low end.

Then it compares that pattern of hits with itself, shifted by every gap between 55 and 210 BPM. Wherever the hits line up best is the beat. A slight preference for tempos between 80 and 160 keeps it from picking a silly answer like 62 for a dance track.

All of this runs on your phone or computer. The song isn't sent to a server, so it works with unreleased mixes, and it works offline once the page has loaded.

How the key is found

For the key, the audio is cut into short windows and each one is broken into frequencies. Every frequency belongs to one of the 12 notes, so the page adds up how strongly each note sounds over the whole clip. That note profile is then compared with the classic Krumhansl–Schmuckler profiles for all 24 major and minor keys, and the closest match wins. The result is shown with its Camelot code (8A, 11B and so on) for harmonic mixing.

When to double-check the result

• Half or double tempo. If the result feels twice as fast or slow as the song, both numbers describe the same beat. The detector lists the other reading under the result.
• Songs that change speed. Live recordings, classical pieces and songs with tempo changes don't have one true BPM. The detector reports the most common one in the section it hears.
• Relative keys. C major and A minor use the same notes, so the key can land on the relative major or minor. The Camelot number stays the same either way, which is what matters for mixing.
• Very quiet or ambient tracks. With no clear hits there's little to measure. If the result says the beat is loose, tap along to confirm.

The Camelot wheel in one paragraph

DJs use Camelot codes because they make harmonic mixing a matter of counting. Tracks with the same code mix cleanly. So do neighbours: one number up or down (8A to 9A) or the same number with the other letter (8A to 8B). Jumps larger than that tend to clash unless the mix is short.

Questions