Bolt Circle / Hole Pattern
Calculate XY coordinates for equally-spaced holes on a bolt circle (BCD). Supports 2 to 36 holes with custom starting angle and center offset.
Introduction
A bolt circle pattern — N holes evenly spaced on a circle of a given diameter — appears on flanges, motor mounts, wheel hubs, and rotary fixtures, and laying one out by hand means trigonometry for every hole. The Bolt Circle Calculator does the whole pattern at once: enter the bolt circle diameter, the number of holes, a start angle, and optionally the circle center's machine coordinates, and it returns the X-Y position of every hole, ready to type into a DRO, a G-code program, or a CMM routine. Each hole also carries its angle, so you can cross-check the pattern against the drawing's angular callouts.
How it Works
Holes are spaced at an angular step of 360 ÷ N degrees starting from the chosen start angle. Each hole sits at its own angle around the circle, and its coordinates are x = cx + (BCD/2) × cos θ and y = cy + (BCD/2) × sin θ — the radius being half the bolt circle diameter. Angles follow the standard convention of 0° at three o'clock increasing counterclockwise, and results are normalized to 0–360°. A 6-hole pattern on a 100 mm BCD starting at 0° therefore lands holes every 60°, the first at (50, 0) from center.
Usage Scenarios
- Drilling a 5-hole pump-flange pattern on a manual mill with DRO: punching in each computed X-Y instead of rotary-table indexing.
- Writing a G-code drilling cycle for an 8-hole motor mount and pasting the coordinate list directly into the program.
- Matching an existing wheel hub: measuring BCD and one hole's angle, then generating the remaining positions for the adapter plate.
- Clocking a pattern: using a 30° start angle so two holes straddle the part's centerline per the drawing note, instead of one hole sitting on it.
- Inspecting a flange on a surface plate by comparing CMM hole centers against the calculator's nominal coordinates.
FAQ
What does the start angle control?
The angular position of the first hole; all others follow at 360 ÷ N increments. Drawings often clock patterns so holes straddle a centerline — a half-step offset like 30° on a 6-hole pattern.
Which angle convention do the results use?
Standard math convention: 0° points along +X (three o'clock) and angles increase counterclockwise, matching most CNC controls and DRO bolt-circle functions. Reported angles are normalized into 0–360°.
Can the pattern be centered somewhere other than the origin?
Yes — enter the circle center's machine coordinates and they are added to every hole position, so the output drops straight into a program without manual offsetting.
How accurate are the computed positions?
Mathematically exact for the given inputs. Real-world pattern accuracy then depends on your machine's positioning, so use the same datum for the center coordinates as the drawing's datum scheme.