The Bolt Circle Diameter (BCD) is the diameter of the circle that passes through the centre of the bolts on your chainring. Many chainrings have the BCD shown on them but if it isn't, you'll need to calculate it.
On a 4 bolt chainring you can simply measure the distance between the centres of opposite bolts. This will give you the BCD for a 4 hole chainring.
For chainrings with 5 bolts, you will need to measure the centre to centre distance between two adjacent bolts (see measurement A in the photo above) and look it up in the table below or use the calculator below. You can use the calculator for any number of bolts as long as you have 'measurement A'.
Note that the outer chainring and inner chainring on a triple crankset will almost certainly have different bolt circle diameters.
Typical BCD for 5 arm spider
|Center to centre between adjacent bolts||Bolt circle diameter|