MusicReasonRhythm, Tempo, Meter, and Practice Timing
How long is this bar, in seconds?
Declare a meter, a tempo with its beat unit, and how many bars you need timed. This tool multiplies out the exact duration and shows the formula it used, without assuming a compound meter's grouping (2+2+2 versus 3+3) changes the total time either way.
Answer
Want to learn more? Tempo, Meter, Beat Units, and Subdivision
Understand
A bar's duration depends on three things only: how many beats it holds (the numerator), how long each of those beats lasts (set by the denominator's note value and the tempo), and how many bars you are timing. Nothing about how a compound meter is felt or grouped changes any of that arithmetic; 6/8 grouped as 3+3 and 6/8 grouped as 2+2+2 last exactly the same total time.
Worked examples
Two bars of 4/4 at 120 BPM last exactly 4 seconds (2 seconds per bar, at 4 beats of 500 ms each). Doubling the bar count to four bars, holding everything else the same, exactly doubles the total to 8 seconds, since duration scales linearly with bar count. A 6/8 bar's BPM is conventionally counted in eighth notes rather than quarters; declaring the correct beat unit here matters for getting the right answer.
Method and limitations
The tool computes the duration of a single denominator-value note from the stated BPM, then multiplies by the numerator for one bar's duration, then by the bar count for the total. BPM is read as counting beats of the bar's own denominator value by default, the ordinary reading of a plain tempo marking; a separately declared beat unit is supported for the less common case where a tempo marking names a different note value than the bar's own denominator. This tool computes exact arithmetic duration only; it makes no claim about how a meter is conventionally felt or grouped.
A 6/8 bar at a tempo marking of "dotted quarter = 100" needs the beat unit declared separately from the bar's own eighth-note denominator, since one dotted-quarter beat actually spans three eighth notes; declaring the beat unit as an eighth note directly and adjusting the BPM figure to match gives the identical bar duration through a different but equally valid input path. Both routes must agree, and this tool's formula holds for either one without silently picking a beat-unit convention for you.