Okay, that’s great but how do I know what my maximum heart rate is? That answer is not so simple and has been elusive for many decades. The most common and equally the most inaccurate way of determining this number is to subtract your age from 220. There are many problems with this equation and can underestimate your maximum heart rate by as much as 35 beats per minute. The theoretical maximum is based solely on your age and does not take into consideration your present fitness level, heart chamber size, or resting heart rate.
Then you can try the Karvonen Method which determines the training heart rate reserve by not only subtracting your age from 220 but also incorporating your resting heart rate. However number 220 is still used to determine the maximum beats/minute this equation so you might as well throw that one away.
A study by Dr. BM Nes from the Norwegian College of Science and Technology and colleagues, published in the Scandinavian Journal of Medicine & Science in Sports showed that researchers tested 3,000 men and women ages ranging from 19 to 89 and determined that the number 211 minus 64% of your age was the most accurate way of selecting your target zone.
This just goes to show you that none of us really know what the magic number is. The conclusion here is to go by the perceived exertion rate of how hard you feel you are exercising. On a scale of 1-10 where 1 is sitting watching a movie and 10 is running up Mount Everest, try and exercise around 6-8. Take your heart rate for for 15 seconds and multiply that number by 4 to give you your heart beats per minute. Most likely this will be your best bet at finding your target heart zone.