Bolt Circle Diameter or BCD is the diameter of the circle that goes through the centre of all of the bolts on your chainring. On bicycle chainring this dimension is usually measured in millimeters. It is critical to know the BCD of your crankset when you are selecting a new chainring for your bike. In many cases the BCD is printed right on the chainring and sometimes it is stamped or engraved on the back side of the chainring.
If it is not labeled on your chainring you will need to measure it by measuring the distance between two adjacent bolts and use the table below to determine the BCD. This is the easiest method to use for chainrings with five bolts. Note that there are a few cranksets that have non-standard arm spacing. If this is the case the distance between adjacent bolts will be different as you move around the chainring. If this is the case standard chainrings will not fit.
4 Bolt Chainrings
5 Bolt Chainrings
The equation for the BCD of any symmetric bolt pattern is:
BCD = d / sin(180/n)
where d is the center to center distance between adjacent bolts, n is the number of bolts, and the angle is in degrees.