Belly putters can make it easier for some golfers to correct errors in their putts, particularly if they're in the habit of twisting their wrists or hips at the end of the swing. For a belly putter to be beneficial, it must be the ideal size. Make a point to measure your personal belly putter length before buying one.
Take an ordinary putter and assume your putting stance. Stand with your feet spread about shoulder width apart and curve your spine downward from your tailbone to the top of your head. Your head should be bowed so your face is parallel with the ground. Your arms should be pointed straight down, and you should hold the putter at such an angle that the head is almost directly beneath your eyes. Visualize a golf ball in this position or use an actual golf ball.
Ask an assistant to use a tape measure to measure a straight line from the head of the putter to your lower abdomen. Make sure the tape measure forms a continuous straight line. If you're assuming the proper stance, the tape measure should hit you a few inches above the waist. For most people, this length is somewhere between 40 and 45 inches.
Go to a golf pro shop that sells belly putters in various sizes. Even though you've measured for proper length, you should try out a belly putter in your size to make sure it feels right. Belly putters can commonly be purchased off the rack in 41-, 42- and 43-inch lengths. Forty-, 44- and 45-inch belly putters are also mass manufactured, but they may be slightly harder to find at retail.
Take a few practice putts with the belly putter of your ideal length, but be sure to also try belly putters that are 1 inch longer and 1 inch shorter than your ideal size. When taking your putting stance with a belly putter, make sure the end of the club touches your abdomen but isn't digging into you. In general, if you're more comfortable with a putter that is a little longer or shorter than your ideal measurement, you should go with the one that is more comfortable.