If you are new to running, or only want an approximation of your maximum heart ate, you can use the formula of 214 – (0.8 x age) for men and 209 – (0.9 x age) for women.
Unfortunately for 5-10 per cent of the population, this figure can be wrong by up to 24 beats per minute. A better way to find your max is to do a running test.
Run evenly as fast as you can for three minutes (ideally on a treadmill), resting with two or three minutes gentle running, then repeating the fast three minutes. During your second run you should get a higher maximum reading than with any other method although you or a helper should use a heart rate monitor to take readings throughout the interval, because your pulse may peak before the finish.