How it works
A bolt circle places N equally spaced holes on a circle of radius
r = BCD / 2
about a center (cx, cy). Hole i (counting from
i = 0) sits at the angle
θi = start + i · (360 / N), measured counter-clockwise from
the +X axis, so its coordinates are
Xi = cx + r · cos θi
and
Yi = cy + r · sin θi.
Adjacent holes are 360 / N degrees apart and a chord
BCD · sin(180° / N) apart in a straight line.
Worked example
A 100 mm BCD with 6 holes starting at
0°, centered at the origin. The radius is 50 mm, the holes
are 360 / 6 = 60° apart, and the chord between neighbours is
100 · sin(30°) = 50 mm. Hole 0 lands at
(50, 0), hole 1 at (25, 43.30), hole 2 at
(−25, 43.30), and so on around the circle. The calculator lists all six
coordinates and renders the pattern.
Frequently asked questions
- How do you calculate bolt circle (BCD) coordinates?
- Put each hole at angle θ = start + i·(360/N) on a circle of radius r = BCD/2, then X = cx + r·cos θ and Y = cy + r·sin θ for hole i = 0…N−1. Enter the diameter, hole count, start angle and center above and the calculator lists every X-Y position.
- What is a bolt circle diameter (BCD)?
- The bolt circle diameter is the diameter of the imaginary circle that passes through the centers of all the holes (bolts) in the pattern. The pattern radius is half of it, r = BCD/2.
- How do I find the hole spacing on a bolt circle?
- The angular spacing between equally spaced holes is 360° ÷ N, where N is the number of holes. For example, 6 holes are 60° apart and 8 holes are 45° apart.
- How do I lay out a bolt pattern on a mill or lathe?
- Set your work-zero at the bolt-circle center (cx, cy), then move to each X-Y coordinate the calculator lists and drill there. On a CNC, the X-Y pairs go straight into the program; on a manual machine, dial them in on the DRO from the center.
- What is the chord distance between adjacent holes?
- The straight-line (chord) distance between two neighbouring holes is BCD·sin(180°/N). For a 100 mm BCD with 6 holes that is 100·sin(30°) = 50 mm — useful for checking spacing with calipers.
- Does this work in metric and imperial?
- Yes — toggle SI/imperial in the header to enter the diameter and center in mm or inches; the coordinates and chord convert with it. The angles stay in degrees.
Method & assumptions
- Holes are equally spaced around the bolt circle; for an unequal pattern, set each angle yourself.
- Angles are measured counter-clockwise from the +X axis; the first hole is at the start angle.
- Coordinates are relative to the chosen center
(cx, cy)— set it to your work-zero. - Round the coordinates to your machine's resolution; the list is shown to 0.001 mm.
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