Bolt Circle Calculator
Lay out evenly spaced holes on a bolt circle. Enter the bolt circle diameter, number of holes, and starting angle to get the spacing between holes and the X–Y coordinates of each hole from the center.
Formula
Hole i: X = (D/2) × cos(θ₀ + i × 360/n), Y = (D/2) × sin(θ₀ + i × 360/n); chord = D × sin(180°/n)
- Holes are placed at equal angular steps of 360°/n around a circle of the given diameter.
- Each hole's position is measured from the circle center, with the start angle setting where hole 1 sits.
- The chord is the straight-line, center-to-center distance between two adjacent holes.
- Coordinates use standard math convention: 0° along +X, angles increasing counter-clockwise.
- Use the same length unit throughout — the results carry whatever unit you enter for the diameter.
100 mm circle, 6 holes, 0° start
Inputs
- Bolt Circle Diameter: 100 mm
- Number of Holes: 6
- Start Angle: 0 °
Six holes are 60° apart. Adjacent holes are 100 × sin(30°) = 50 mm apart, and hole 1 sits at (50, 0) mm.
Frequently asked questions
What is a bolt circle?
A bolt circle (or bolt pattern) is a set of holes spaced evenly around an imaginary circle, used for flanges, wheels, and mounting plates.
How is the hole spacing found?
The center-to-center distance between adjacent holes is the chord length, D × sin(180°/n), where D is the diameter and n the number of holes.
What is the start angle for?
It sets where the first hole is placed. Changing it rotates the whole pattern without changing the spacing.
What units should I use?
Any consistent length unit. The coordinates and spacing come out in the same unit you enter for the diameter.
Are the coordinates measured from the center?
Yes. Each X–Y pair is relative to the center of the bolt circle, with positive X to the right and positive Y up.