Bolt Circle (PCD) Calculator
Enter the pitch circle diameter, number of holes and start angle to get every hole's X/Y coordinates — with a diagram and CAD-ready exports.
Hole Pattern
Angles in degrees. Coordinates use the same unit as the PCD.
Angle of hole 1.
Optional — for the diagram, clearance check and exports.
Diagram
Summary
Hole coordinates
| Hole | Angle | X | Y |
|---|
Find the PCD from a Part
Measure between two neighbouring hole centres to work out an unknown bolt circle.
Bolt Circle Formulas
The maths behind the calculator, and the conventions it follows.
Hole Angle
θᵢ = start + (i − 1) × 360° / nHoles are spaced equally around the circle, starting from hole 1 at the start angle.
Hole Position
X = Xc + R·cos θ · Y = Yc + R·sin θR is half the PCD. Xc and Yc are the bolt circle's centre — leave them at 0 for coordinates relative to the centre.
Neighbour Spacing
chord = PCD × sin(180° / n)The straight-line distance between adjacent hole centres — the number to check with a vernier.
Finding an Unknown PCD
PCD = chord / sin(180° / n)For an even number of holes you can also measure straight across opposite hole centres — that is the PCD.
Straddling the Centreline
start = 180° / nFlanges usually have holes straddling the vertical and horizontal centrelines rather than sitting on them.
CAD Conventions
0° = +X · anticlockwiseAutoCAD, MicroStation and most CAD systems measure angles from 3 o'clock, anticlockwise. Change these if your drawing uses 12 o'clock or clockwise.
Ligament
chord − hole ØThe material left between neighbouring holes. If it's zero or less, the holes overlap.
Using the Exports
.csv · .scr · .dxf · .svgCSV opens in Excel. Drag the .scr onto AutoCAD to draw the holes. DXF opens in almost any CAD program.
Why BoltCircle?
BoltCircle is a fast, lightweight calculator for drafters, CAD designers, machinists and engineers who need accurate hole positions on a pitch circle — without setting up a drawing first.
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