Insert your thumb fully in the thumb hole to determine if your bowling grip is conventional, fingertip or semifingertip. You have a conventional grip if your pointer and middle fingers are inserted up to the second knuckle. If your pointer and middle fingers are inserted to the first knuckle, you have a fingertip grip. And if your pointer and middle fingers are inserted only a short amount between the tip and first knuckle, you have a semifingertip grip.
Measure 1/4 inch behind the knuckle — where the ball hits your pointer and middle fingers — according to your grip style. For example, if you have a fingertip grip, measure 1/4 inch back from the first knuckle on your pointer and middle fingers. The additional 1/4 inch is added to take into account the curve of the bowling ball.
Mark your fingers with an ink pen or marker at the measurement area on each finger.
Place your thumb in the appropriate hole in the bowling ball fitting tool. Place your first middle finger, or pointer finger, on the measuring tool and note the number that corresponds with the mark on your finger. This distance is your finger span. Repeat the process with your middle finger.