Bolt Circle Calculator

A bolt circle is a set of equally spaced holes on an imaginary circle, defined by the bolt circle diameter (BCD) and the number of holes. The angle between holes is 360 degrees divided by the number of holes, and each hole sits on the circle at that spacing.

From the BCD and hole count you get everything: the angle between holes, the straight-line (chord) spacing between neighbors, the arc spacing along the circle, and the X/Y coordinate of each hole.

For example, a 4 inch bolt circle with 6 holes spaces them 60 degrees apart, with a 2 inch chord between neighbors. This calculator gives the angles, the spacing and every hole coordinate for any BCD and hole count.

Each hole’s coordinate comes from the radius and its angle around the circle. With the center at Cx, Cy and the radius equal to half the BCD, a hole at angle theta is at X = Cx + R times the cosine of theta, and Y = Cy + R times the sine of theta.

The angle of each hole is the start angle plus its index times 360 divided by the number of holes, measured counter-clockwise from the 3 o’clock direction. Leaving the center at zero gives coordinates relative to the circle center.

For example, on a 4 inch bolt circle with 6 holes starting at 0 degrees, the first hole is at X 2, Y 0 and the next at X 1, Y 1.732. This calculator lists the X/Y of every hole, ready to lay out or program.

The center-to-center spacing between two neighboring holes is the chord of the bolt circle — the straight-line distance across the circle between them, not the curved distance. It is the BCD times the sine of 180 degrees divided by the number of holes.

This is the dimension you measure with calipers between adjacent hole centers, so it is the handiest check that a pattern is laid out right.

For example, a 4 inch bolt circle with 6 holes has a chord of 4 times the sine of 30 degrees, which is 2 inches. This calculator reports the chord spacing alongside the arc and the angle between holes.

Both measure the gap between neighboring holes, but along different paths. The chord is the straight-line distance between two adjacent hole centers. The arc is the curved distance along the bolt circle itself between the same two holes.

The arc is always a little longer than the chord, and it equals the circumference divided by the number of holes, or pi times the BCD divided by the number of holes. The chord is what you measure with calipers; the arc is the share of the circle each hole takes.

For example, a 4 inch, 6-hole bolt circle has a 2 inch chord and a 2.094 inch arc between neighbors. This calculator gives both so you can measure or lay out whichever way suits.

For an even number of holes, the bolt circle diameter is simply the center-to-center distance between two holes directly across from each other, because opposite holes lie on a diameter. Measure between those two hole centers.

For an odd number of holes there is no hole straight across, so measure from the center of one hole to the midpoint between the two holes opposite it, or measure the chord between adjacent holes and back-calculate. A simpler route is to measure a known chord and let the geometry give the BCD.

For example, on a 6-hole pattern, the distance across between opposite holes is the 4 inch BCD. This calculator works from the BCD (or radius); confirm it against the mating part or drawing.

The start angle sets where the first hole sits, measured counter-clockwise from the 3 o’clock (positive X) direction. It rotates the whole pattern without changing the spacing: 0 degrees puts the first hole at 3 o’clock, and 90 degrees puts it straight up at 12 o’clock.

It matters for lining the pattern up with a feature, a keyway or the mating part, and for whether a hole lands on an axis or between them. The angle between holes stays 360 divided by the count regardless.

For example, a 6-hole pattern at a 0 degree start has a hole at 3 o’clock; at a 30 degree start the holes shift so a gap faces 3 o’clock instead. This calculator lets you set any start angle and rotates the coordinates with it.

They describe the same circle, one as the full width and one as the half. The bolt circle diameter (BCD) is the distance across the circle through the center; the radius is from the center out to a hole, which is half the BCD.

Drawings and standards usually quote the diameter — a wheel might be 5 on 4.5, meaning 5 holes on a 4.5 inch BCD — but the coordinate math uses the radius, so the calculator converts between them.

For example, a 4 inch bolt circle diameter is a 2 inch radius. This calculator lets you enter whichever your drawing gives and uses the radius internally for the hole coordinates.

No. The angle between holes, the start angle and each hole’s angular position depend only on the number of holes, not on the unit. A 6-hole pattern is 60 degrees apart whether you work in inches or millimeters.

Only the lengths — the BCD, the chord, the arc, the circumference and the X/Y coordinates — carry a unit and change their numeric value between systems. The angles are pure.

This calculator works in inches or millimeters and keeps all the lengths in that unit, while the angles stay the same. Just keep the BCD and center coordinates in the same unit.