What half-time and double-time mean
Half-time and double-time describe a feel, not a change of tempo. In a half-time feel the groove seems to move at half the speed while the underlying tempo stays the same. A double-time feel is the opposite, with the groove seeming to move twice as fast.
How half-time sounds
In a half-time groove the snare often lands on beat 3 instead of beats 2 and 4, so the music feels slower and heavier even though the tempo has not changed. You hear this in rock, hip-hop, metal, pop and electronic music, among other styles.
Why your BPM reads double or half
A tap tempo tool measures whatever you tap. One person taps the fast pulse and another taps the slow, heavy one, and both are describing the same music. Software that detects BPM automatically has the same problem, because it can lock onto the pulse it hears most clearly.
An example
Say you tap along with a track and the reading settles at 70 BPM, but the song feels like it is moving at 140. Both numbers describe the same music. At 70 BPM one beat lasts about 857 ms, and at 140 BPM it lasts about 428 ms. The BPM to ms calculator gives you either one.
How to switch between them
Press the ½× button above to halve the reading, or 2× to double it. If you are setting a project tempo or a delay time, pick one count and stay with it. If you are playing along, use whichever count feels natural.
Common questions
Is half-time the same as slowing a song down?
No. The tempo stays the same and only the feel changes.
Is double-time faster?
It feels faster, but the tempo has not changed. If the music really speeds up, that is a tempo change.
Which BPM should I use?
Either can be right. Choose the count that suits what you are doing, and keep it consistent.