A simple formula and one number on your wrist tell you if your workout counts.

PRINTED ON TREADMILLS and exercise bikes in gyms around the world is a simple method for estimating the maximum rate at which your heart should safely beat, in beats per minute: 220 minus your age.

Any form of cardio, like running, raises your heart rate. But how high is too high? If you look at your heart rate monitor and notice your bpm (beats per minute) has climbed to, say, 170, you should ...