A bolt circle, or pitch circle diameter (PCD), is the circle that passes through the centre of every bolt hole on a flange, hub or wheel. Enter the number of holes and the PCD below to get the X and Y position of each hole and the straight-line spacing between neighbours. If you are measuring an existing part, enter the spacing between two adjacent hole centres instead and the calculator works the PCD back for you.
Bolt circle calculator
Bolt circle diameter
--
Adjacent hole spacing: --
Angle between holes: --
Units are whatever you enter: millimetres in, millimetres out. Coordinates are measured from the centre of the bolt circle, X to the right and Y upwards, with angles counted anticlockwise from the X axis.
Hole spacing factors for common bolt patterns
The straight-line distance between two neighbouring holes is the PCD multiplied by the sine of 180 degrees divided by the number of holes. The table gives that factor for common patterns, and its inverse for working back from a measurement.
| Holes |
Angle between holes (deg) |
Spacing factor (spacing = PCD x) |
PCD factor (PCD = spacing x) |
Spacing on a 100 mm PCD (mm) |
| 3 | 120 | 0.8660 | 1.1547 | 86.60 |
| 4 | 90 | 0.7071 | 1.4142 | 70.71 |
| 5 | 72 | 0.5878 | 1.7013 | 58.78 |
| 6 | 60 | 0.5000 | 2.0000 | 50.00 |
| 8 | 45 | 0.3827 | 2.6131 | 38.27 |
| 10 | 36 | 0.3090 | 3.2361 | 30.90 |
| 12 | 30 | 0.2588 | 3.8637 | 25.88 |
| 16 | 22.5 | 0.1951 | 5.1258 | 19.51 |
| 20 | 18 | 0.1564 | 6.3925 | 15.64 |
| 24 | 15 | 0.1305 | 7.6613 | 13.05 |
How to measure PCD on an existing part
With an even number of holes the job is simple: measure centre to centre across two opposite holes and that distance is the PCD. Measuring from the near edge of one hole to the far edge of the opposite hole gives the same answer when both holes are the same size, and it is easier to do with calipers.
With an odd number of holes there is no opposite hole to measure to. Measure centre to centre between two adjacent holes and multiply by the PCD factor in the table. For five holes that factor is 1.7013, so adjacent holes measured at 58.78 mm apart sit on a 100 mm bolt circle. The same method works for any hole count, which makes it the safer habit on parts you cannot identify.
Check the result against a standard before drilling. Pipe flanges follow fixed bolt circles for each nominal size and pressure rating, listed in the flange PCD chart, so a measured value that lands a millimetre away from a standard figure is almost always that standard figure plus measuring error.
Worked example: 6 holes on a 100 mm PCD
Six holes sit 60 degrees apart. The radius is 50 mm, so each hole is at X = 50 x cos(angle) and Y = 50 x sin(angle). The spacing between neighbours is 100 x 0.5 = 50 mm, which is the one pattern where the spacing equals the radius.
| Hole |
Angle (deg) |
X (mm) |
Y (mm) |
| 1 | 0 | 50.000 | 0.000 |
| 2 | 60 | 25.000 | 43.301 |
| 3 | 120 | -25.000 | 43.301 |
| 4 | 180 | -50.000 | 0.000 |
| 5 | 240 | -25.000 | -43.301 |
| 6 | 300 | 25.000 | -43.301 |
The formulas
Angle between holes = 360 / N
Adjacent hole spacing = PCD x sin(180 / N)
PCD = adjacent hole spacing / sin(180 / N)
Hole position: X = (PCD / 2) x cos(angle), Y = (PCD / 2) x sin(angle)
N is the number of equally spaced holes. On a milling machine with a digital readout, set the datum at the centre of the bolt circle and drive to each X and Y pair in turn. On a rotary table or dividing head, index by the angle between holes instead.
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