BPM & Key Finder

Tempo and key, read from the waveform

Most players only repeat what the file claims in its tags, and those tags are often missing or wrong — a 128 BPM track still labelled 120 because nobody updated it. This tool ignores metadata entirely and measures the audio.

• Tempo comes from the onset envelope: detect where the energy jumps, then find the repeating interval that explains those jumps.
• Key comes from chroma: every frequency is folded onto one of twelve pitch classes, and the resulting profile is matched against major and minor key templates.
• Both run on a downsampled copy of two minutes of audio, which is why the answer arrives in about a second.

Why the BPM sometimes looks doubled

Tempo detection has one classic ambiguity: 87 BPM and 174 BPM produce the same pulse, because every beat of the slower reading is also a beat of the faster one. Drum and bass, trap and half-time tracks are the usual suspects.

The large number is the reading that fits the beat pattern best, but the runner-up values sit right next to it. If a track feels twice as fast as the number suggests, the doubled value is almost always in that list. Trust your ears: for mixing, what matters is that two tracks share a pulse, not which label you use.

What a key name actually tells you

A key is a home base — the set of notes a track keeps returning to, and the note it feels resolved on. Knowing it lets you transpose a vocal, pick a harmonically compatible track for a transition, or work out which chords will sit under a melody.

Results are written the way musicians write them: root note plus major or minor. C major and A minor contain the same seven notes; the difference is which one feels like home, which is why both are reported instead of a bare set of notes.

• DJs: mixing in key usually means moving between neighbouring keys on the Camelot wheel rather than jumping across it.
• Producers: check whether a sample is in the key of the track before pitching it.
• Players and singers: transpose a song into a key your voice can reach.

How accurate is it, honestly?

On steady electronic, pop and rock tracks the tempo is usually exact or within a decimal. On music with rubato, live drums, spoken word or long ambient passages it can drift, and a cappella intros can pull the key estimate toward whichever note dominates.

The confidence bars are there for a reason: a short gap between the top two candidates means the material really is ambiguous, not that the tool failed. Treat anything under about 40% as a hint rather than a fact.

Formats, limits, and what happens to your file

Anything the browser can decode works: MP3, WAV, M4A/AAC, OGG, Opus and most FLAC files. Decoding uses the Web Audio API, so format support matches your own browser.

The file is read from disk into this tab and analysed in place. Nothing is uploaded, nothing is stored, and closing the tab leaves no trace. No account, no quota, no watermark.

• At least 4 seconds of audio is required to produce a result.
• The two minutes starting at the 15% mark are analysed; longer files are trimmed, not rejected.
• Silent or near-silent recordings are rejected rather than guessed at.

Frequently asked questions

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