How to Determine the BCD for your Crankset/Chainrings
BCD stands for Bolt Circle Diameter. It deals with the measurement for chainrings. You need the proper BCD to make sure the chainrings match the crankset (when you are replacing chainrings.)
Since the BCD probably isn't stamped on your crankarm, you need to take a measurement and do a calculation.
1. Measure center-to-center distance between two adjacent chainring bolts, as shown in the picture. In millimeters (mm)
2. Multiply by what I call the CRF - chainring factor. If you have a 4-bolt spider, multiply by 1.4. With a 5-bolt spider, multiply by 1.7.
The number you get should be close to a common BCD.
For example, I was measuring one of my Shimano mountain bike cranks one day since I needed to replace the big ring. I measured the distance to be about 72.5 mm, and I multiplied by 1.4 since it's a 4-bolt crank. 72.5 x 1.4 = 101.5, so the chainring must be a 104 BCD. So an "FC-M540" ring would fit.
To see a list of common BCD numbers and the corresponding cranks, take a look at this chart from Sheldon Brown.