The song is decoded and analysed by your own browser. Nothing is uploaded.
How the tempo is found
The tool listens for the moments where the sound suddenly gets louder or brighter, such as drum hits and plucked notes, and builds a pulse curve from them. It then looks for the time gap that repeats most strongly through the song and turns it into beats per minute between 60 and 200. Many songs can be counted at half or double speed, so both are shown; pick the one that matches how you would clap along.
How the key is found
The sound is split into its frequencies and folded into the twelve notes of the scale, so the tool knows how much of each note the song contains. That note profile is compared with the well-known Krumhansl-Schmuckler profiles for every major and minor key, and the closest match wins. The relative key shares the same notes, and the Camelot code helps DJs find songs that mix smoothly: neighbouring numbers, or the same number with A and B swapped, usually blend well.
How accurate is it?
These are estimates. They work best on music with a steady beat and a clear tonal centre, such as pop, dance, rock and hip-hop. Songs that change tempo or key, live recordings that drift, and ambient or very percussive tracks are harder, and the confidence word drops to tell you so. If a result looks off, try the tap tab and trust your ears.
Tap tempo
Tap the button, the space bar or the T key in time with the music. The tempo is the average of your most recent taps, so it settles after a few beats. Stop for two seconds and the next tap starts a fresh count.