Bolt Circle Calculator

A bolt circle, also called a pitch circle diameter or PCD, places hole centers at equal angular intervals around an imaginary circle. This layout is common on flanges, wheels, sprockets, covers, pipe connections, and machined plates. The calculator converts a circle diameter and a bolt count into angular spacing and straight-line spacing between adjacent centers. It also reports the Cartesian coordinate of the first center from a user-selected starting angle. Angular spacing is found by dividing a full 360-degree revolution by the number of bolts. Six holes therefore have 60 degrees between centers, while eight holes have 45 degrees. The adjacent chord is not the same as the arc length around the circle. It is the direct center-to-center distance and equals the pitch circle diameter multiplied by the sine of half the angular spacing. This dimension is useful for inspection with calipers and for checking a drawing. Coordinates are measured from the center of the bolt circle. The radius is half the entered diameter. The X coordinate is the radius multiplied by the cosine of the angle, and the Y coordinate is the radius multiplied by the sine. Angles begin on the positive X axis and increase counterclockwise. Every following hole adds one angular-spacing increment to the starting angle. Tiny displayed values near zero are rounded by the calculator for readability. Use one consistent unit for the pitch circle diameter and all resulting linear dimensions. If the diameter is entered in millimeters, the chord and coordinates are millimeters. If it is entered in inches, those results are inches. Angular values always remain in degrees. The calculator describes nominal hole-center geometry and does not add hole diameter, positional tolerance, edge distance, or manufacturing allowance. For fabrication, verify that the bolt count, PCD, orientation, and datum convention match the engineering drawing. Confirm that holes have adequate clearance and that the surrounding material can carry the intended load. Coordinate output can support layout or CNC programming, but machine coordinates may require a different origin, clockwise angle convention, or axis orientation. Apply the appropriate transformation before using the values in production.