How it works
Holes on a bolt circle sit evenly around a circle centered on your part origin. Divide 360° by the hole count for the angular step, then convert each angle to X and Y from the center using cosine and sine.
Enter the circle diameter, the number of holes, and the angle of the first hole. The table can be typed straight into a CNC program or a layout sheet.
Worked example
Six holes on a 4.5 inch bolt circle starting at 0°:
- Angle between holes = 360° ÷ 6 = 60°
- Chord between neighbors = 4.5 × sin(180° ÷ 6) = 2.25 in
- Each hole: X = 2.25 × cos θ, Y = 2.25 × sin θ, with θ = 0° + i × 60°
| Input | Value |
|---|---|
| Bolt circle diameter | 4.5 in |
| Number of holes | 6 |
| Start angle (from +X axis) | 0 ° |
| Result | Value |
|---|---|
| Chord distance between adjacent holes | 2.25 in |
| Angle between holes | 60 ° |
| Hole | Angle (°) | X (in) | Y (in) |
|---|---|---|---|
| 1 | 0 | 2.25 | 0 |
| 2 | 60 | 1.125 | 1.9486 |
| 3 | 120 | -1.125 | 1.9486 |
| 4 | 180 | -2.25 | 0 |
| 5 | 240 | -1.125 | -1.9486 |
| 6 | 300 | 1.125 | -1.9486 |
Assumptions and limits
- Angles are measured counter-clockwise from the positive X axis with the origin at the circle center.
- Holes are evenly spaced. Uneven patterns need each angle entered separately.
- Coordinates are shown to four decimal places; check the precision your machine and tolerance require.
Common questions
What is a bolt circle (PCD)?
It is the diameter of the circle passing through the centers of a set of holes, also called the pitch circle diameter.
How do I measure a bolt circle with an odd number of holes?
Measure from one hole edge to the farthest opposite hole edge and use the calculator to convert; or measure center-to-center between adjacent holes and solve for D = chord ÷ sin(180° ÷ N).
How do I rotate the pattern?
Change the start angle. A 5-hole pattern starting at 90° puts one hole at the top.
Sources
- Plane geometry: points on a circle of radius R at equal angular spacing; chord = D sin(180° ÷ N).
Updated 2026-09-30