Listen to the song and determine the beat of the song. The rhythmical beat is the steady tempo that occurs throughout the song, but can change with each movement of the song. Once you have determined the beat, tap your foot to keep the beat going.
Start your stopwatch at the exact time where one of the beats starts. If you are tapping your foot, the beat begins when your foot taps the ground. Time yourself for 20 seconds and count the number of beats that occurs within the 20 seconds.
Multiply the number of beats you counted within the 20-second period by three to give you the tempo of the song. For example, if you counted 50 beats in 20 seconds, the tempo of the song would be 150 BPM.