BPM Detector

How the BPM Detector works

Tempo sounds like the simplest fact about a song, yet it is one of the hardest to pin down by ear. A shuffle feel, a half-time chorus or a slow ballad with busy hi-hats can all trick a listener into counting at twice or half the real rate. The BPM matters any time audio has to line up with something else. You need it to set a DAW project to a sample's tempo, cut video to the beat, match two tracks in a DJ set, or set a delay in milliseconds.

Where tempo turns up. DJ software shows the BPM of every track so the pitch fader can bring two songs together. Beatmatching a 124 BPM house track to a 128 BPM one needs about a 3 % speed change, which most listeners will not notice. Video editors cut on the beat because a cut that lands a frame or two early looks deliberate, while one that lands late looks like a mistake. Fitness playlists are sorted by tempo: roughly 120–140 BPM for a brisk walk or spin class and 160–180 for running cadence. The CPR guidelines tell rescuers to compress at 100–120 per minute, which is why Stayin' Alive at about 104 BPM is the song trainers name. Genres sit in familiar bands, too: hip-hop around 85–95, house 120–128, techno 125–140, drum and bass 170–175.

How it works. The selected region is mixed to mono and turned into an onset envelope. This is a trace, a hundred samples per second, of how sharply the level is rising, so it spikes on every drum hit and note attack. The tool then compares the envelope with copies of itself, shifted by every lag between the highest and lowest tempo you allow. The shift where the spikes line up best is the beat period. With the period known, it tries every offset to find where the beats actually fall. That gives the first beat and, from there, the position of every beat in the region.

Half and double. Autocorrelation cannot tell a beat from a half bar or from a pair of eighth notes, because the envelope lines up just as well at 2×, ½× and sometimes 3× the true tempo. The Lowest and Highest tempo limits are the main defence. Narrow them to the range the genre suggests, such as 60–100 for a ballad or 160–180 for drum and bass, and the octave error has nowhere to hide. Arguments over whether a track is 87 or 174 BPM are really about whether you count the kick or the hi-hat. For a DJ both are right, but a metronome or a delay setting needs you to choose one.

Selecting a region. Only the selected range is analysed, so pick a section with a steady beat and a clear drum part. A chorus works better than a free-time intro. Give it at least ten seconds; twenty to thirty is better. If you analyse the whole of a track that changes tempo, it will report one of the two tempos with low confidence. Split the track and measure each part separately.

Tap tempo. Press play, then tap the T key on each beat. The tool averages the gaps between taps and shows the result next to the detected value. A pause of more than two seconds starts a new count. Eight to sixteen taps are enough to settle a half-or-double question, and tapping is the fastest way to check a low-confidence result.

Reading the BPM detector's result

• Tempo is the best estimate, to one decimal place. Studio tracks made to a click usually land within 0.1 of a whole number. A value like 97.3 suggests a live performance or a recording that has been sped up or slowed down.
• Confidence measures how far the best candidate stands out from the next one that is not a close neighbour. A steady electronic track often scores above 60 %. Below 20 % the tool adds a warning, and the estimate should be treated as a guess until tapping confirms it.
• Half / double lists the octave alternatives. If the headline figure feels too frantic or too sluggish, one of these is the tempo a musician would count.
• Runner-up is the strongest candidate outside the half/double family. When it sits at about 1.5× or 0.67× the tempo, the track probably has a triplet or dotted feel.
• First beat gives the time of the first detected beat and roughly how many beats the region contains. Use it to set a DAW's grid offset.
• Beat length gives the seconds per beat and per four-beat bar. Multiply by 1000 for a quarter-note delay in milliseconds, then halve it for an eighth note.

Pitfalls. Heavy sidechain pumping, long reverb tails and sustained pads blur the onset envelope. Music without drums, such as solo piano or string quartets, often has no steady pulse at all, and a low confidence score there is an honest answer, not a fault. Swing pulls the off-beats late, so compare the swing and straight readings before you quantise. A file that has been time-stretched keeps its groove but not its original BPM.

Exporting. Click track renders a stereo file with the song at half volume and a short click on every beat from the first beat to the end. Every fourth beat is higher in pitch so the bars are audible. Listen all the way through: if the clicks drift, either the tempo is slightly off or the performance is not on a grid. Beat list writes a CSV with the tempo, confidence, first beat time and beat length, followed by the time of every beat. Import it as markers or use it to drive cuts in a video editor.

Related tools