BPM

BPM is beats per minute: how many pulses fit into a minute of music. It is the oldest and most reliable number in the booth, because you can count it without having an opinion.

The usual trap is half time. A drum and bass track at 174 and the same track read as 87 are the same music: one program counted the pulse and another counted every second one. So a BPM difference that looks enormous may not be one, and a fifty per cent jump between two consecutive tracks is almost always this rather than a mistake in the order. The compatibility checker reports it separately instead of counting it as a jump.

For ordering a set, tempo is useful in two ways. As a proxy for energy, because in most genres faster feels higher — a real relationship but a loose one, which the engine uses as a starting point rather than a verdict. And as a mixing constraint: two tracks far apart in tempo can be brought together, but it costs something, and that is where pitch comes in.

If your tracks carry no BPM in their tags, there is no guessing it from a filename. They have to be analysed by a program that reads the audio.