Bolt Circle Calculator
Use this bolt circle calculator to generate evenly spaced hole coordinates around a circular bolt pattern with an optional starting angle and center offset.
Bolt Circle Coordinates
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Quick Answer: Each hole on a bolt circle sits at x = x_c + R cos(theta) and y = y_c + R sin(theta), where theta advances by 360 / n degrees around the pattern.
How to Calculate
- Enter the hole count: Set the number of equally spaced holes on the circle.
- Enter radius and offsets: Provide the bolt-circle radius and any center offset you want to add.
- Choose the first-hole angle: The starting angle rotates the whole pattern around the center.
- Read the coordinate table: The calculator reports each hole's angle and x-y coordinate pair.
Formula
x_i = x_c + R cos(A + i x 360 / n), y_i = y_c + R sin(A + i x 360 / n)
| Variable | Meaning | Unit |
|---|---|---|
| R | Bolt-circle radius | length |
| A | Starting angle of the first hole | degrees |
| n | Number of holes | count |
| x_c, y_c | Center offsets | length |
Worked Examples
Flange drilling - Four-hole bolt circle
- Holes: 4
- Radius: 50 mm
- Start angle: 0 deg
Result: Hole coordinates fall at the four cardinal points around the circle
Changing the start angle rotates the whole pattern without changing spacing.
Interpretation Table
| Range | Meaning | Action |
|---|---|---|
| More holes | Smaller angle step between holes | Confirm minimum edge distance and drill clearance. |
| Larger radius | Holes spread farther from the center | Check part diameter and flange thickness. |
| Non-zero offset | Pattern center is shifted | Verify datum references before machining or drilling. |
Frequently Asked Questions
A bolt circle is a circular pattern of equally spaced holes used for flanges, hubs, and other fastened parts.
The starting angle rotates the entire pattern so the first hole lines up with your chosen datum direction.
Yes. It provides each hole's angular position and its x-y coordinates on the selected pattern.
Note: This calculator provides layout geometry only. Final manufacturing should also account for hole diameter, tolerance, clearance, and drawing standards.
References
Last reviewed: March 14, 2026