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Measuring Bicycle Crank and Chainring Bolt Circle Diameter
Original article: https://www.sheldonbrown.com/measure-bcd.html on sheldonbrown.com
- Reproduced with the permission of John Allen and Harriet Fell — all rights reserved. The version on sheldonbrown.com is authoritative.
- Mirrored: 2026-10-11 · Last update of the original: see original
Use this page in connection with our bolt-circle diameter cribsheet, which identifies the major brands and models associate with each bolt-circle pattern
No two of an odd number of bolt holes are directly opposite another. Also, a protruding part may prevent direct measurement of the bolt-circle diameter of a crank spider. So, the cribsheet lists spacings between bolt holes which are not opposite one another. These can be measured with a ruler or caliper. The table at the bottom of this page gives the bolt-circle diameters that correspond to the measurements.
Many references describe distances between bolt holes as "center to center," but there is only empty space at the center of a hole! D'oh! Measuring from right side to right side or left side to left side is more reliable -- as shown in the photo below.
Measuring between non-adjacent holes reduces the likelihood of mistakes when differences are small. As an exercise, you may check the dimensions in the table on our cribsheet page to identify the chainring in these photos, and then double-check against the brand-name stamp on the chainring.
Some newer chainrings have unevenly-spaced holes. You need to measure between more than one pair of holes to identify these. Details are on another page.
