Chord Length Calculator

The formula

Chord length = 2 × R × sin(included angle ÷ 2), where R is the circle's radius and the included angle is measured at the circle's center between the two points. For evenly spaced holes on a bolt circle, the included angle between any two adjacent holes is simply 360° ÷ number of holes.

Why this beats measuring with a DRO alone

Not every machine has a readout, and even with one, a caliper check across two already-drilled holes is a fast, independent way to confirm a bolt circle came out right — if the measured chord between adjacent holes matches what this calculator predicts, the spacing is correct without needing to trust the DRO or program alone.

A hexagon is a good sanity check

For exactly 6 evenly spaced points on a circle, the chord between adjacent points always equals the radius exactly — a useful fact to sanity-check this calculator, or any bolt circle layout, against with almost no math at all.