How to Use a Bolt Hole Circle Calculator

— Precise CNC Hole Pattern Design Guide

TL;DR – Quick Answer

A Bolt Hole Circle (BHC) is a circular arrangement of holes evenly spaced around a center point, used for mounting flanges, gears, wheels, and more.

Calculate bolt hole spacing on a circle with this formula:
Spacing = 2 × Radius × sin(180° ÷ Number of Holes)

Or simply use EKINSUN’s Bolt Hole Circle Calculator for instant, error-free results.

What is a Bolt Hole Circle?

A Bolt Hole Circle arranges multiple holes equally spaced along the circumference of a circle with a fixed diameter, called the Pitch Circle Diameter (PCD).

This geometry is critical in mechanical assemblies for:

Mounting wheels and gears
Flange connections in piping
Robotics and aerospace parts

Step-by-Step: Calculating Bolt Hole Circle Parameters

Step 1 – Determine PCD:
Measure or define the Pitch Circle Diameter — the diameter of the imaginary circle the holes sit on.

Step 2 – Count Number of Holes:
Decide how many holes you need around the circle.

Step 3 – Calculate Spacing Between Holes:
Use:
Hole Spacing = 2 × Radius × sin(180° ÷ Number of Holes)
Where Radius = PCD ÷ 2

Step 4 – Use Calculator for Ease:
Input PCD and hole count in our Bolt Hole Circle Calculator to get spacing, hole coordinates, and drill path instantly.

Example Use Case: Flange Design

PCD = 100 mm
Number of holes = 8

Calculate hole spacing:

Radius = 100 ÷ 2 = 50 mm
Spacing = 2 × 50 × sin(180° ÷ 8) = 2 × 50 × sin(22.5°) ≈ 38.27 mm

This spacing ensures your flange bolts align perfectly.

Manufacturing and Design Tips

Verify Tolerances: CNC machines typically achieve ±0.05 mm accuracy on hole placement.
Consider Bolt Size: Hole diameters should allow clearance for bolt shanks and washers.
Account for Material: Harder materials may require slower drilling speeds and extra care.
Label Coordinates: If sharing CAD files, clearly indicate hole positions and center points.

When to Use a Bolt Hole Circle Calculator

Designing flanges, pulleys, and wheels
Creating mounting plates for motors and gearboxes
Fabricating robotic joint assemblies
Any part requiring precise circular hole patterns

Terminology to Know

Pitch Circle Diameter (PCD): Diameter of the circle passing through hole centers
Bolt Circle: Synonym for PCD in many industries
Hole Spacing: Chord length between adjacent holes on the circle
Chord Length: Straight-line distance between two holes, not arc length

Who Should Use This Tool?

Mechanical and manufacturing engineers
CNC machinists and programmers
Product designers using CAD/CAM software
Educators teaching mechanical design concepts

Frequently Asked Questions About Bolt Hole Circles

Q: How is PCD measured?
A: PCD is the diameter of the circle on which the centers of the holes lie. Use calipers or CAD software for accuracy.

Q: What if holes aren’t evenly spaced?
A: This tool assumes equal spacing. For irregular patterns, manual design or specialized software is needed.

Q: Can I calculate hole coordinates?
A: Yes, many bolt hole circle calculators (including ours) provide XY coordinates for CNC programming.

Q: Is there a minimum hole spacing?
A: Maintain at least 2× hole diameter between holes to avoid material weakness.

Q: Can this tool work for metric and imperial units?
A: Yes, simply input your units consistently.

👤 About the Author

Alex Liao is the lead engineer at EKINSUN, with over 10 years of experience in CNC machining, materials engineering, and product design. He specializes in simplifying technical workflows for global engineers and manufacturers. His team has delivered precision parts for aerospace, robotics, and medical sectors across 12+ countries.

🔗 Visit EKINSUN’s Engineering Team
📧 For technical inquiries: support@ekinsun.ltd

Related Tools

About EKINSUN

EKINSUN delivers precision CNC services and engineering tools designed to help professionals create accurate parts faster. Our free online calculators simplify your engineering workflow.

Contact Us

Have questions or need custom CNC parts? Visit our Contact Page or upload your CAD files for a prompt quote.

Google Sites

Report abuse