Park Tool CDG-2 Chainring Diameter Gauge
The CDG-2 easily and accurately measures bolt circle diameter (BCD) in millimeters for a wide variety of four- and five-bolt chainring patterns. Made of anodized aluminum and permanently laser engraved, the tool centers itself in chainring holes, chainring bolts, or crank arm holes. The CDG-2 also includes a center-to-center distance scale and a handy reference chart listing additional mounting pattern measurements.
I can eyeball the BCD with a tape measure anchored to one bolt and a good guess at where the one directly opposite would sit. So can you. There aren't so many BCD's out there that a few mm's off of your guess will cause problems. You'll guess 129 and realize that size doesn't exist and know that it's a 130. My point is that this tool is a bit overkill. It works, but so does a tape measure.