Each hole sits at X = r cos(angle), Y = r sin(angle), with the angle stepping by 360 / holes. Six holes on a 2 inch circle starting at 0 degrees land at (1.0000, 0), (0.5000, 0.8660), (-0.5000, 0.8660), (-1.0000, 0), (-0.5000, -0.8660), (0.5000, -0.8660).
Approach every hole from the same direction on each axis to take backlash out of the dials.
Coordinates are from the circle center. Chord distance between adjacent holes = diameter x sin(180 / holes), useful to check the layout with a caliper.
Tools for this job
The tooling this calculation is usually for, spread across the price range.
How does the bolt circle calculator work?
A bolt circle is the classic job that a DRO makes easy and a mill with only handwheel dials makes careful. Either way the numbers are the same trigonometry. Measure every position from the circle center, and always approach from the same side to cancel backlash.
Frequently asked questions
How do I drill a bolt circle without a DRO?
Pick up the circle center with an edge finder, zero the dials, then move to each coordinate by counting handwheel turns, always approaching from the same direction so backlash is taken up the same way. Center drill every hole first, then drill. A rotary table does the same job by rotation instead of coordinates.
Keep going
Formulas are standard shop math; starting speeds are conservative conventions for hobby machines. Not professional engineering advice. Wear eye protection and keep hands, gloves and sleeves clear of rotating work.