Count the number of beats in 15 seconds
Multiply that number by 4
Count the number of beats in 30 seconds
Multiply that number by 2
Count the number of beats in 10 seconds
Multiply that number by 6
Use a heart rate monitor or pulse app for an automatic BPM reading
For music, count the beats in one minute
For music, divide 60 by the seconds per beat to get BPM