You are in a record shop. Something is playing. You need to know whether it will sit next to the other thing you are holding.
Or you are in a booth at one in the morning and the incoming track is close but you cannot tell if it is close enough.
Either way the question is the same and it takes four seconds to answer: what tempo is this?
Tap the beat
That is the whole interface. A large pad, and the space bar if you are at a desk. Tap along with the kick. After three taps it starts reading, and by eight it is solid.
It averages the intervals between your taps, and it forgets any tap older than six seconds. That second part is not a limitation, it is how you reset: stop tapping, wait a moment, start again on the next track. Nothing to clear.
It tells you where you are, not just what you have
A number on its own is only half useful. 128 means something specific; 174 means something else entirely; 142 is a genuine fork in the road.
So it names the territory as well. Dub and downtempo below 80. Hip-hop up to 100. That awkward, brilliant 100-to-118 stretch where UK garage lives. House. Tech house. The 140 corridor where dubstep and grime argue. The jungle approach at 146. Drum and bass properly at 160 and up. And above 182 it stops being helpful and asks if you are all right.
Those boundaries are opinions. They are the opinions of somebody who has spent a lot of time on both sides of a mixer, and they are more useful than no opinion.
There is no microphone
This matters, so it is on the page as well as here: TAP does not listen to anything. It cannot. It counts the intervals between your finger and nothing else.
Every "BPM detector" that listens to a room needs microphone permission, and microphone permission on a phone in a club is a genuinely bad trade for a number you can produce yourself by tapping. Counting taps is less clever and more honest, and in a loud room with a bassy PA it is frequently more accurate too.
Tapping well
Tap the kick, not the melody — melodies lie, and syncopation will have you reading double time.
Give it eight taps rather than four. The maths is an average, and averages get better with more of them.
And if the number comes out at exactly half or exactly double what you expected, you are tapping the wrong layer. Everyone does it. Halve it, double it, carry on.