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
32.9mm 56mm
34.1mm 58mm
43.5mm 74mm
50.3mm 86mm
55.3mm 94mm
64.7mm 110mm
69.4mm 118mm
76.4mm 130mm
79.4mm 135mm
84.6mm 144mm

BDC calculator


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