Start the music and begin tapping the beat with your foot. Don't confuse the beat with the rhythm. The beat is the steady rhythmic pulse in the music. It is the beat that you normally dance to, or walk in time to.
Start the stopwatch and count the number of beats within twenty seconds of time. Write down this number for future reference.
Multiply the number you received by three. This will give you the total beats-per-minute for the song. You may use a calculator if it helps.