This calculator provides a bolt circle
diameter (BCD) measurement from the distance between two adjacent bolt holes. The bolt
holes can be measured center to center or back to back so long as the the
corresponding distance type is selected on the calculator. This calculator is
most useful for sprockets with an odd number of bolt holes or for sprockets with
an even number of bolt holes that are mounted on a wheel where there is an axle
or rim hub that gets in the way of measuring the sprocket's bolt circle
diameter.

How To Measure Adjacent Bolt Hole Center To Center Distance
Step 1  Measure Bolt Hole ID and Zero the
Digital Caliper
Step 2  Measure Adjacent Bolt Hole Back To Back Distance
Step 3  The Digital Caliper Readout Is Now the
Adjacent Bolt Hole Center To Center Distance
Measuring the adjacent bolt hole center to center distance
or back to back distance and using the bolt circle diameter calculator
located above or using a mathematical formula is the only way to determine
the bolt circle diameter of sprockets, rims, axle hubs, and brake rotors
with an odd number of bolt holes or with an even number of bolt holes where
there is an axle or rim that gets in the way of measuring the diametrically
opposed bolt holes to determine the bolt circle diameter.
How To Measure Bolt Circle Diameter
Step 1  Measure Bolt Hole ID and Zero the
Digital Caliper
Step 2  Measure Diametrically Opposed Bolt Hole Back To Back Distance
Step 3  The Digital Caliper Readout Is Now the Bolt Circle Diameter
Only the bolt circle diameter (BCD) of an even number of
bolt holes can be measured this way. For sprockets, rims, axle hubs, and
brake rotors with an odd number of bolt holes or with an even number of bolt holes where there
is an axle or rim that gets in the way of measuring the diametrically
opposed bolt holes the adjacent bolt hole distance will need to be measured and the bolt circle diameter calculator
located above used to determine the bolt circle diameter.