Flanges, wheels, motor mounts and pipe fittings use bolt holes evenly spaced around a circle. On a mill or drill press you need the coordinates of each hole relative to the centre. The first hole can start at any angle – at 0° (3 o’clock) or rotated so holes straddle the centreline.
How to use the Bolt Circle Calculator
- Number of holes
- Bolt circle diameter
- Angle of first hole
- Centre X
- Centre Y
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the number of holes, the bolt circle diameter, the angle of the first hole and the centre coordinates.
Tips for an accurate estimate
- Measure angles with a digital angle finder or sliding bevel, not by eye.
- Make a test cut in scrap before cutting expensive stock.
- Small angle errors add up over many joints – check the first assembly before cutting the rest.
- Keep a written list of the dimensions and angles you set on the saw or machine.
Common mistakes to avoid
- Reading the saw scale as the angle from the edge rather than from square.
- Rounding intermediate results too early.
- Mixing degrees and percent grade.
Inputs explained
| Field | What to enter |
|---|---|
| Number of holes | A number |
| Bolt circle diameter | Units: mm, in, cm |
| Angle of first hole | Units: °, rad |
| Centre X | Units: mm, in, cm |
| Centre Y | Units: mm, in, cm |
What each result means
- Distance between adjacent holes = [Bolt circle diameter]×sin(rad(180÷[Number of holes]))
- Angle between holes = 360÷[Number of holes]
Example values
How the main result changes when Number of holes changes and everything else stays at the example values above:
| Number of holes | Distance between adjacent holes (mm) |
|---|---|
| 3 | 86.603 |
| 4.5 | 64.279 |
| 6 | 50 |
| 7.5 | 40.674 |
| 9 | 34.202 |
| 12 | 25.882 |
Formula
Angle of hole k = start angle + (k − 1) × 360° ÷ nX = Xc + D/2 × cos(angle); Y = Yc + D/2 × sin(angle)Chord spacing = D × sin(180° ÷ n)Worked example
Inputs — Number of holes: 6; Bolt circle diameter: 100 mm; Angle of first hole: 0 °; Centre X: 0 mm; Centre Y: 0 mm.
Results — Distance between adjacent holes = 50 mm; Angle between holes = 60 °; Hole 1 – X = 50 mm; Hole 1 – Y = 0 mm; Hole 2 – X = 25 mm; Hole 2 – Y = 43.301 mm; Hole 3 – X = -25 mm; Hole 3 – Y = 43.301 mm; Hole 4 – X = -50 mm; Hole 4 – Y = 0 mm; Hole 5 – X = -25 mm; Hole 5 – Y = -43.301 mm; Hole 6 – X = 25 mm; Hole 6 – Y = -43.301 mm.
Key terms
- PCD
- Pitch circle diameter (bolt circle diameter).
Frequently asked questions
How do I calculate bolt hole positions?
X = R cos θ and Y = R sin θ for each hole angle θ, added to the centre coordinates.
What is the distance between bolt holes?
D × sin(180°/n): for 6 holes on a 100 mm circle, 50 mm.
What does PCD mean?
Pitch circle diameter – another name for the bolt circle diameter.
What formula does the Bolt Circle Calculator use?
Angle of hole k = start angle + (k − 1) × 360° ÷ n.