CNC / machinist calculator
Bolt Circle Calculator
Lay out a bolt hole circle without trigonometry by hand. Enter the bolt circle diameter, the number of holes and the angle of the first hole, and this calculator returns the exact X and Y position of every hole, ready to key into a DRO or a G-code program. You can offset the circle to any centre, so the coordinates come out in the same part zero you are already working from on the machine.
Coordinates are in the same unit as the diameter, measured from the center you enter. Angles run counter-clockwise from the positive X axis.
How it works
Holes on a bolt circle are spaced evenly around a circle of a known diameter. The radius is half the bolt circle diameter, and the holes are spaced by 360 degrees divided by the number of holes. Each hole sits at its own angle, and its position is the centre plus the radius times the cosine of that angle in X and the radius times the sine of that angle in Y.
Angles here are measured counter-clockwise from the positive X axis, the standard convention that matches a mill table with X to the right and Y away from you. The start angle rotates the whole pattern, so set it to place the first hole where the print calls for it, for example 90 degrees to put a hole straight up at twelve o'clock.
Reading coordinates straight off the calculator avoids the rounding errors that creep in when you chain angle additions by hand, and it makes odd hole counts such as five or seven as easy as a four-hole flange.
Worked example
Four holes on a 4 in bolt circle starting at 0 degrees land at (2, 0), (0, 2), (-2, 0) and (0, -2). Set the start angle to 45 degrees to rotate the whole pattern between the axes.
Frequently asked questions
What is a bolt circle diameter?
The bolt circle diameter, or BCD, is the diameter of the imaginary circle that passes through the centres of all the holes in the pattern. The holes are spaced evenly around that circle at its radius.
How do I find the coordinates of bolt holes?
Place each hole at the bolt circle radius and at an angle of 360 divided by the hole count times its index. The X is the centre plus radius times cosine of the angle, and the Y is the centre plus radius times sine.
How does the start angle change the pattern?
The start angle rotates the entire hole pattern around the centre. Setting it to 90 degrees puts the first hole straight up; setting it to zero puts the first hole out to the right along the positive X axis.
Can it handle odd numbers of holes?
Yes. The spacing is simply 360 degrees divided by the number of holes, so five, six or seven holes are no harder than four. The calculator lists every hole coordinate regardless of how the spacing divides.
Why use coordinates instead of a rotary table?
Coordinates let you drill a bolt circle on any mill with a DRO or in a CNC program without a rotary table or indexing head. They also avoid the cumulative angle errors that come from setting each hole by hand.
Related calculators
Sources
Every formula on this page is shown and sourced. See how we verify.
These calculators are for planning and as a starting point. Recommended speeds and feeds are published starting values that vary with your specific tool, coating, machine rigidity, workholding and coolant. Always start conservative, listen to the cut, and follow your tool maker data sheet.