Bolt Circle Calculator
Laying out bolt holes on a circular pattern is a common machining and fabrication task. Enter the bolt circle diameter (BCD), number of bolts, and starting angle to get exact X/Y coordinates for each hole, plus the spacing between them.
How Bolt Circle Layout Works
A bolt circle is defined by its diameter (BCD) and the number of evenly spaced holes around it. The center of each hole sits on the circle, and the angular spacing between holes is simply 360 divided by the number of bolts.
Key formulas:
- Angular spacing = 360 / number of bolts
- Chord length = 2 x radius x sin(180 / number of bolts)
- X position = radius x sin(angle)
- Y position = radius x cos(angle)
Common Bolt Circle Patterns
- 4-bolt: 90 degrees apart, used on smaller flanges and wheel hubs
- 6-bolt: 60 degrees apart, common on pipe flanges and truck wheels
- 8-bolt: 45 degrees apart, heavy-duty flanges and large machinery
- 12-bolt: 30 degrees apart, large industrial flanges
Measuring an Existing Bolt Circle
If you need to find the BCD of an existing part, measure the center-to-center distance between two adjacent bolts. Then use: BCD = distance / sin(180 / number of bolts).