Bars to seconds calculator
A bar lasts beats-per-bar divided by tempo, times sixty. At 120 BPM in 4/4 a bar is exactly two seconds, so eight bars is sixteen.
How to use the bars to seconds calculator
That 120 BPM coincidence. Two seconds a bar, thirty bars a minute; is why so much dance music sits there or near it: the arithmetic is trivial and sections land on round numbers. It matters for anything timed against picture. A 30-second advert is exactly 15 bars at 120 in 4/4, and if your track is at 128 it is 16 bars with a second left over, which is the sort of thing that gets discovered late. Worth noting for video work that this assumes constant tempo; anything with a tempo map needs the sections added up separately.
Questions
Exactly two seconds in 4/4. Thirty bars to the minute.
Fifteen at 120 BPM in 4/4. At 128 BPM it is sixteen bars and a bit.
Yes. A bar of 3/4 at the same tempo is three quarters the length of a bar of 4/4.
No, it assumes a constant tempo. Add the sections separately for a tempo map.
Partly the arithmetic — round bar lengths make arranging and syncing to picture much easier.