Bolt Circle Calculator
The positions of equally spaced holes on a bolt circle — the angle of each, the straight-line distance between neighbours, and the x–y coordinates from the centre — from the pitch circle diameter, the number of holes and the angle of the first.
Holes on a pitch circle sit at equal angles: 360° over the count.
How the bolt circle calculator works
Holes on a pitch circle sit at equal angles: 360° over the count. Each hole's position is the radius times the cosine and sine of its angle, measured from the centre, which is what a mill or a drill press with a table needs. The chord between neighbours — twice the radius times the sine of half the pitch angle — is what a divider or a tape measures between hole centres.
Formula: θᵢ = θ₀ + 360° × i / n; xᵢ = R cos θᵢ; yᵢ = R sin θᵢ; chord = 2R sin(180°/n)
Worked examples
| Inputs | Chord between adjacent holes | Note |
|---|---|---|
| Six holes on a 120 mm circle | 60 mm | chord 60 mm |
| A five-stud wheel | — | the 5 × 114.3 pattern |
| Starting at 90° | 60 mm | first hole at the top |
FAQFrequently asked questions
What is a bolt circle?
A circle through the centres of a set of evenly spaced holes — flanges, wheels, pump mounts. Its diameter is the pitch circle diameter, and the count and start angle fix every hole.
How do I measure a bolt circle on a part?
For an even count, the distance across opposite hole centres is the PCD. For an odd count, measure between adjacent centres and divide by the sine of half the pitch angle — the chord row in reverse.
What are the coordinates for?
Setting out on a milling machine, a CNC program or a drill-press table with dials: each hole is an x and a y from the centre. Rotate the start angle to match the part's datum.
Why is the chord less than the arc?
A straight line between two points on a circle is shorter than the curve. Dividers and tapes measure the chord; the arc only matters for marking with a wheel.
What is 5 × 114.3?
The commonest car wheel pattern: five studs on a 114.3 mm (4.5 in) pitch circle. Wheel fitment is that number plus the centre bore and offset.
Where these figures come from
- The Engineering ToolBox — material densities, coverage and unit data — bulk densities, concrete at 2,400 kg/m³, board and sheet sizes
- Cement Concrete & Aggregates Australia — concrete basics and yields — bag yields, density 2,400 kg/m³
- Australian Building Codes Board — the National Construction Code
Last checked: September 2026. Coverage rates and material sizes are typical manufacturer figures — check the product you buy.