BPM and key detector, with nothing to upload

Knowing a track's tempo and key is the step before almost everything else: building a DJ set that flows, writing a counter-melody that doesn't clash, dropping in a sample without it dragging, or just setting the metronome before you record. It's thirty seconds of information that takes ten minutes to work out by hand.

There are three ways to get it here, depending on what's in front of you. If you have the file, drop it in and you get BPM and key. If what's playing is a live band, a video or your own rehearsal, tap the pulse out with tap tempo. And if you want to know which exact note is sounding, the tone detector hears it through the mic. The file, on top of that, isn't uploaded anywhere: it's decoded in your browser and the whole analysis happens on your machine.

The doubled BPM: why it happens and how to settle it

Every automatic detector gets caught by a factor of two sooner or later, and it's worth understanding why before trusting one blindly. The algorithm looks for periodicity in the signal's energy, and plenty of tracks have two equally valid periodicities: the kick and the snare. Drum and bass at 174 has a perfectly audible half-time at 87, and neither number is false — it depends on what you're counting.

The quick check is to count four beats out loud over the song and see whether they match the number. If you're counting twice as fast as the music feels, halve it. And if it still isn't clear, hit tap tempo while it plays: your own pulse is the tie-breaker.

The key and its relative

Key detection compares the song's distribution of notes against the statistical profile of every major and minor key. It works well with sustained harmony and fails on the predictable things: drums alone, an a cappella vocal, a static pad. The most common error isn't random, it's systematic — mistaking a key for its relative, because A minor and C major have exactly the same seven notes and differ only in where the music comes to rest.

If you're mixing two tracks by key, that distinction matters less than you'd think: relatives are compatible with each other, so an error of that kind won't break your transition. Where it does matter is when you're writing on top.

Measuring is one thing; hearing it is another

A tool hands you the number, but the medium-term goal is not needing it: recognising a tempo by ear and knowing what key you're in without looking. Tempo sense trains by playing in time, which is exactly what Beat Drop is, and recognising notes and keys is one of the skills in ear training. For a tuned reference to compare against, there's the online piano.

Frequently asked questions

Is my file uploaded to a server?

No. The audio is decoded in your own browser and the analysis runs on your machine; the file never leaves your device and isn't stored anywhere. That's the difference from most online detectors, which need to upload the track to analyse it.

Which formats does it accept?

MP3, WAV, AIFF, OGG and M4A. It analyses the first three minutes, which is plenty to pin down a song's tempo and key; if you drop in something longer than ten minutes it warns you that it's only looking at the beginning.

Why does it give me half or double the real BPM?

That's the classic failure of any automatic detector, and it isn't always wrong: a track at 140 with the snare on every other beat has a real pulse of 70, and both readings are defensible. If the result feels off, try doubling or halving it and check against the track using tap tempo.

How reliable is the detected key?

It does well on clearly tonal material with sustained harmony. It struggles with anything low on harmonic information — drums alone, a cappella vocals, ambient textures — and it often confuses a key with its relative, since they share the same notes. When you're torn between A minor and C major, listen for where the song comes to rest.

Why use tap tempo if it already detects a file's BPM?

For everything that isn't a file: getting the tempo of something playing live, of a video, of a rehearsal, or checking how fast you're actually playing. It averages the last eight taps, so it settles within four or five bars, and you can hit the space bar instead of the mouse.

And the tone detector — how is that different?

It listens through your microphone and tells you the note sounding right now, with its octave, its frequency in hertz and how confident it is in the reading. It's for a single note or a monophonic instrument; for the key of a whole song, use the file tab.