Bolt Circle Calculator
Estimate the location of holes in a circular pattern using the bolt circle calculator.
Angle between holes60°360° ÷ 6
Chord between adjacent holes2 inmeasure this to check the layout — 50.8 mm
Bolt circle radius2 in
Bolt circle diameter4 in101.6 mm
Holes6
First hole at0°
Chord across two spaces3.4641 inuseful when adjacent holes are too close to measure
Across opposite holes4 inwith an even count, opposite holes span the full diameter
Hole coordinates
| # | Angle | x | y |
|---|---|---|---|
| 1 | 0° | 2 | 0 |
| 2 | 60° | 1 | 1.73205 |
| 3 | 120° | -1 | 1.73205 |
| 4 | 180° | -2 | 0 |
| 5 | 240° | -1 | -1.73205 |
| 6 | 300° | 1 | -1.73205 |
Coordinates are in inches from the centre given, with x to the right and y upward.
The formula
hole i at angle 360° i ÷ n; chord = 2 R sin(180° ÷ n)
Measure the chord, not the angle
Setting out a bolt circle by angle needs a rotary table or a DRO. Checking it is far easier: the straight-line distance between adjacent holes is 2R sin(180°/n), and a caliper across two holes will find an error a protractor would miss.
With an even number of holes, opposite pairs span the full diameter, which gives an even simpler check. With an odd number there is no opposite pair, so the chord measurement is the only practical one.