BPM finder & key finder

How to find the BPM of a song

There are two ways, and they work best together. Automatic detection: drop an audio file onto the left panel. Within a second or two you get the tempo, the key, the Camelot code DJs use for harmonic mixing, and a confidence reading. Tap tempo: if the music is playing somewhere you can’t grab a file from — a radio, a club, a video, a band rehearsal — tap the big pad (or press the space bar) on every beat. After four to eight taps the reading settles.

Whichever you use last fills the tempo box above the delay table, so you can go from “what tempo is this?” to “what delay time do I need?” without retyping anything. Hit Open drum machine at this tempo to start a beat at the song’s BPM with the 808 kick already tuned to the song’s key.

Half time, double time and why detectors disagree

Tempo is partly a matter of perception. A trap beat can be counted at 140 BPM, with the snare on beat three, or at 70 BPM with the snare on two and four — both are “correct”. Drum & bass is written at around 174 BPM but many listeners nod along at 87. Automatic detectors, including this one, have to choose a level, so xx808 prefers tempos between roughly 70 and 180 BPM and checks whether the beats in between are as strong as the beats it picked. If you disagree with the choice, the Half time and Double time buttons switch instantly and update everything downstream.

Ranges are typical, not rules — plenty of records sit outside them.

How the detection works

The file is decoded by your browser, mixed to mono and resampled. For tempo, the audio is split into short overlapping slices (about 6 ms apart) and the energy in 36 frequency bands is compared slice to slice; sudden increases mark note and drum onsets. The resulting “onset strength” curve is then compared with time-shifted copies of itself: when the shift equals one beat, the peaks line up and the match is strongest. The best match, refined to a tenth of a BPM, becomes the tempo.

For key, the analyzer measures how much energy falls on each of the twelve pitch classes (C, C♯, D … B) across the whole song and compares that profile with the classic Krumhansl–Kessler profiles for the 24 major and minor keys, which come from listening experiments on how strongly each note “belongs” in a key. The closest match wins, and the runner-up gap becomes the confidence figure. Keys that share the same notes — like C major and A minor — are the hardest to tell apart, so the relative key is shown as well.

Delay and reverb times from BPM

The table under the tool converts the current tempo into note lengths in milliseconds. A quarter note lasts 60,000 ÷ BPM milliseconds: 500 ms at 120 BPM. An eighth note is half that, a sixteenth a quarter. Dotted values are 1.5 times longer and triplets two-thirds as long. Set a delay plug-in to the 1/8 dotted value for the classic rhythmic echo, or use the 1/16 or 1/32 value as a reverb pre-delay so the dry hit stays clear before the tail arrives. The hertz column is handy for syncing an LFO or tremolo by ear on gear without tempo sync.

Tips for accurate results

• Songs with a steady drum beat give the most reliable tempo. Rubato piano, ambient music and live recordings that speed up will read as an average.
• For key detection, choose a section with harmony — a long drum intro or an a cappella breakdown can mislead it.
• When tapping, tap on the kick and snare rather than the hi-hats, and keep going for at least eight taps.
• If the result looks wrong, check the candidate list under the chroma bars: the alternative tempos the detector considered are listed there.