Take a look at the palm of your left hand. If you look closely, you will see three major and distinct lines.
The lower line (palm readers call this your LIFE LINE) starts at the base of your hand where your thumb connects to your wrist. Follow this line and you will see that it curves around the base of your thumb and ends up near the base of your index finger.
There is also an upper line (palm readers call this your HEART LINE) that starts below your pinkie finger and runs across your palm to the base of your index finger.
Between these two is a middle line that starts between your index finger and thumb and then heads across your palm before vanishing about three-quarters of the way across. Palm readers call this your HEAD LINE. This is the line that is used for measuring tennis grip size.
Using a ruler, measure the distance between the tip of your middle finger and the HEAD LINE immediately below your middle finger. That is your grip size. When choosing a tennis racquet, this is the maximum size you should consider.
A grip larger than this will make if difficult to generate power on your serve. A grip smaller than this will lead to tennis elbow.
Remember that a slightly smaller grip can be made to fit by adding an additional layer of grip or overgrip on the racquet handle. However, a grip size cannot be made smaller. So if you must purchase a racquet that is not the perfect size, choose one that is up to a quarter inch smaller than perfect.