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The Holes
You need either a bow compass or dividers (available fairly cheaply in geometry sets) or an internal caliper or a piece of wood and some wood drills.
With the Bow Compass, Dividers or Internal Calipers:- You need to find probably three house balls. One that fits your thumb comfortably, one that fits your ring finger comfortably, one that fits your middle finger comfortably. You then measure each of these holes using the bow compass, dividers or internal calipers by putting it in the hole about 3cm down and expanding it to fit. You take your hole diameter measurement from across the widest point near the bottom of the compass or across the points of the dividers or internal calipers.
With the Drills:- Simply drill the holes in the block of wood and find the ones that comfortably fit your thumb, middle finger and ring finger. Record the drill size for thumb, middle finger and ring finger.
The Spans
 
Left Hand Right Hand
For Conventional Drilling
Using the bow compass, dividers or internal calipers you can CAREFULLY use the points to measure the distance from the base of the thumb (the crease where the thumb joins the palm in the V) to the middle of the second crease in your middle finger (the first crease is where your finger starts after your palm). Then measure from the base of the thumb to the middle of the second crease in your ring finger. This is shown on the line drawing above but do not spread your fingers as shown. Keep them together with your thumb out. Take 3mm or 1/8" off both measurements. The ring finger measurement is usually, in 95% of cases, slightly longer than the measurement for the middle finger. You don’t see this on house balls as they are generically drilled for use by both right and left handed bowlers.
Alternatively, and probably preferably, you can use house balls to find the right span for each finger. Just find a ball for the middle finger where the thumb is all the way in and the middle finger goes into the finger hole at 90 degrees and is not over or under spanned. (Don’t worry about the hole sizes here as they are not relevant). When measuring the distance (or span) between the two holes try to ignore the bevel (rounding at the top of the hole) and measure from the inside of the thumb hole to the inside of the finger hole in each case. Do the same for the ring finger. There is no need to take 3mm off these measurements.
It is not unusual for it to require five house balls to obtain a complete personal measurement.
From these measurements I will calculate the rest of the drilling variables required for your customised bowling ball when you send your measurements on ordering the Custom Drilling and your ball.
For Fingertip Drilling
It is the same procedure as the conventional drilling except you measure from the base of the thumb to the middle of the final (third) crease on the middle and ring fingers and then take off 6mm (or 1/4") from each measurement. Unfortunately house balls can't help most people here as the spans are too short.
Measuring Your Hand For A Preset Drilling.
Measure your thumb using one of the above methods and then pick the preset drilling with that thumb size
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