Calculators
Bolt Circle Calculator
Enter the number of holes, bolt circle diameter, and start angle — get the exact X/Y coordinate of every hole (inch or mm), clockwise or counter-clockwise, with a printable coordinate table and a scaled diagram for the shop floor.
Last verified: 2026-07-09
A bolt circle (or bolt circle diameter, BCD) is a set of holes evenly spaced around a circle. To drill or program them you need each hole's X/Y position from the circle's center. Enter the pattern below to get the coordinates and a diagram — switch between inch and millimetre, set the start angle, and choose clockwise or counter-clockwise numbering.
Hole coordinates (mm)
| Hole | Angle (°) | X (mm) | Y (mm) |
|---|---|---|---|
| 1 | 0 | 50 | 0 |
| 2 | 60 | 25 | 43.301 |
| 3 | 120 | -25 | 43.301 |
| 4 | 180 | -50 | 0 |
| 5 | 240 | -25 | -43.301 |
| 6 | 300 | 25 | -43.301 |
X/Y are measured from the bolt-circle center (0, 0). Angle is from +X (3 o'clock), counter-clockwise.
Coordinates are measured from the bolt-circle center, with the angle taken from the +X axis (3 o'clock). Hole 1 sits at your start angle; the rest are spaced evenly by 360° ÷ number of holes in the chosen direction. Add a center offset if your program's origin isn't at the bolt-circle center.
Frequently asked
- How do I find bolt circle diameter if I can't measure it directly?
- For an even number of holes, the BCD is the straight-line distance between two opposite holes (center to center). For an odd number, measure from one hole's center to the midpoint of the gap between the two holes opposite it, or measure across two adjacent holes and use the spacing to back out the diameter.
- Which way is a positive start angle?
- Angles are measured from the +X axis (3 o'clock). A start angle of 90° puts the first hole straight up (12 o'clock). Counter-clockwise increases the angle; clockwise decreases it — use the direction toggle to match your drawing or G-code.
- Can I use this for a flange or wheel pattern?
- Yes — any evenly-spaced circular hole pattern (flanges, wheels, faceplates, index plates). Enter the hole count and BCD; the coordinates and diagram follow.
Sources
- Exact circle geometry (X = R·cos θ, Y = R·sin θ) about the bolt-circle center