The most accurate way of measuring Maximum Heart Rate for an individual is via a cardiac stress test. In such a test, the athlete exercises while being monitored by an EKG (An electrocardiogram). During the test, the intensity of exercise is periodically increased. The test normally last for around 10-20 minutes. This test requires the sports equipment and a trained person to run it and as such can be expensive. However there have been a number of studies and numerous formulas for predicting the Maximum Heart Rate. Including the most quoted one "HRmax=220-age" equation.
ROBERT A. ROBERGS AND ROBERTO LANDWEHR did a study on all the formulas know in 2002.
They came to the conclusion that the most accurate formula for general population was one by Inbar { HRmax = 205.8 − (0.685 × age) }although it still had a large margin of error of 6.4 beats/minute when applied to an individual athlete. Therefore these formulas should be used as a guide only.
As such the result in this calculation presents an Estimate of your Maximum Heart Rate (HRmax) and in addition it adds the margin of error of 6.4 b/min to give an upper and lower limit. Chances are pretty good your HRmax will be within this range.