I have wondered for a while how chainrings are sized. Even though I have been riding for years, I have never needed to replace one so I never realised how many different sizes and shapes there are.
The number of teeth on the chainring and the number of bolts (4 or 5) is pretty easy to choose. This depends on what type of riding you doing and what type of cranks you have. The measurement that isn’t so easy is the Bolt Circle Diameter, otherwise known as the BCD, of the chainring.
The size that you need depends on the groupset and is sometimes written on the face of the chainring. If it is not, then the table below will help you determine what size chainring you need.
Using a tape measure, measure the distance between the center of 2 adjacent holes. (See the picture) Then use that measurement to determine what size chainring you need depending on if you have a 4 or 5 hole crank.
I hope this helps prevent multiple trips to the bikeshop with the incorrect chainring size.
Greg
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