How the tempo is found
The song is mixed down to mono and the tool looks for the moments where the sound suddenly gets louder in some part of the spectrum: drum hits, plucked notes, the start of a chord. It then measures how strongly that pattern repeats at every tempo from 60 to 200 BPM. A song at 128 also repeats at 64 and 256, so the tool decides between them, preferring 80 to 160 unless the music clearly says otherwise, and gives half and double as well. Last, a grid of evenly spaced beats is fitted to the whole song, which pins the tempo to a tenth of a BPM and lines up the click you can play over it.
How the key is found
Every note in the song, from 50 Hz to 5 kHz, is folded onto the twelve pitch classes (all the Cs together, all the C♯s, and so on), after allowing for a recording tuned a little off A440. That profile is compared with how often each note is heard in music in each of the 24 major and minor keys, and the closest match wins. You get the key, its Camelot code and Open Key code, the relative key that shares its notes, and the runner-up. Keys next to each other on the Camelot wheel (8A, 8B, 7A, 9A) mix smoothly.
Checking it by ear
Play with clicks plays the song with a click on every beat it found, so you hear at once whether they sit together. If the clicks run at twice or half the speed of the beat you feel, pick Half or Double; that tempo goes into the table and the CSV. Tap along measures your own taps as on Tap Tempo and says which reading they agree with. Copy as CSV copies the whole table for a spreadsheet or a DJ library.
Questions
How accurate is it?
On music with a steady beat, such as dance, pop, hip hop and rock, the tempo is usually right to within a BPM, though it can land on half or double. Key finding is harder for everyone: songs that change key, modal or very sparse music, and tracks that sit on one chord can come out as the relative key (A minor for C major) or a fifth away, which is why the runner-up and a confidence are shown. Live recordings that speed up give an average.
Is my music uploaded?
No. Each file is decoded and analysed inside this page, on your device, in a background worker, so a long batch does not freeze the page. Nothing is sent anywhere and nothing is kept after you close it.
Which files work?
Whatever your browser can play: MP3, WAV, M4A and AAC, OGG and Opus, and FLAC in current browsers. Safari cannot open OGG Vorbis. Listening through the microphone records 15 seconds and analyses that; it works best close to a speaker in a quiet room.
What are Camelot and Open Key?
Two ways of numbering the keys round the circle of fifths so that compatible keys sit next to each other. Camelot runs 1 to 12 with A for minor and B for major (A minor is 8A, C major 8B). Open Key, used by Traktor, runs 1 to 12 with m and d (A minor is 1m, C major 1d).