So why this calculator? Well no other reason then it might be another fun one to add. I had found the formula online and implemented it. Here's it is for inquiring minds (or incase it's wrong!)
To Find the Bolt Circle from measuring adjacent studs use this formula: Bolt Circle = BDist / sin(180/n) Where- Bolt Circle = The Diameter of the bolt pattern BDist = The distance CENTER TO CENTER of adjacent lugs n = Number of bolts in the pattern
Note : The sine function sin(), in this case must expect the value of (180/n) in degrees not radians.
The calculator below accepts a few parameters and has the option to allow you to measure from lugs centers or lugs back to back (as you might do with a caliper). This will work with Metric (mm) or Imperial (inches), or Parsecs for that matter.
If you want one of the slick looking tools to make the measuring easier just check out Amazon, they have a few and they are pretty Inexpensive Here are the links (opens in a new window as to not lose your place here) -