Bolt Circle Calculator

Calculate the coordinates and chord lengths for bolts arranged in a circular pattern for flanges, wheels, and mechanical assemblies.

Last updated: March 2026

Bolt Circle Diameter (BCD) (in)

Number of Bolts

Start Angle (Degrees)

Bolt Layout

Chord Length (Bolt-to-Bolt)

5 in

Bolt 1(5, 0)0°

Bolt 2(2.5, 4.3301)60°

Bolt 3(-2.5, 4.3301)120°

Bolt 4(-5, 0)180°

Bolt 5(-2.5, -4.3301)240°

Bolt 6(2.5, -4.3301)300°

Disclaimer: This calculator provides geometric bolt pattern layouts for reference and planning purposes only. Always verify dimensions and tolerances against engineering drawings before machining, fabrication, or assembly. Precision requirements may vary by application.

What is a Bolt Circle?

A bolt circle (or bolt circle diameter, BCD) is a pattern of holes or bolts arranged in a circle. It's commonly used in mechanical engineering for flanges, wheels, gears, and other rotating components where multiple fasteners must be evenly spaced around a central axis.

The Bolt Circle Diameter (BCD) is the diameter of the imaginary circle that passes through the center of each bolt hole. The chord length is the straight-line distance between the centers of two adjacent bolts. Knowing these dimensions is essential for drilling hole patterns, measuring existing assemblies, and ensuring proper fitment of replacement parts like bicycle chainrings, automotive wheels, and industrial flanges.

How to Calculate Bolt Circles

Coordinate Calculation

X Coordinate = R × cos(Angle)

Y Coordinate = R × sin(Angle)

Angle Step = 360° ÷ Number of Bolts

Where R is the radius (half the BCD). Each bolt is positioned at a multiple of the angle step, starting from the specified start angle. Coordinates are given in a Cartesian system with the center at (0, 0).

Chord Length Calculation

Chord Length = 2 × R × sin(180° ÷ N)

Where R is radius and N is the number of bolts. This formula calculates the straight-line distance between adjacent bolt centers, useful for layout and verification.

Example: 6-Bolt Flange

Calculate positions for a 6-bolt pattern on a 10-inch diameter circle, starting at 0°:

Given:

BCD = 10 inches, Bolts = 6, Start Angle = 0°

Step 1:

Calculate angle step:

360° ÷ 6 = 60° between each bolt

Step 2:

Calculate radius:

R = 10 ÷ 2 = 5 inches

Step 3:

Calculate chord length:

2 × 5 × sin(180° ÷ 6) = 2 × 5 × sin(30°) = 10 × 0.5 = 5.0 inches

Result:

Chord = 5.0", Bolts at 0°, 60°, 120°, 180°, 240°, 300°

Frequently Asked Questions

What is BCD?

BCD stands for Bolt Circle Diameter. It's the diameter of the imaginary circle that passes through the center of all the bolt holes in the pattern.

How do I measure BCD on a 5-bolt wheel?

For a 5-bolt pattern, measure from the center of one bolt to the back of the second bolt over. This shortcut is commonly used by mechanics and bike mechanics.

Does the start angle matter?

The start angle determines the orientation of the entire bolt pattern. 0° is typically on the positive X-axis (3 o'clock position). Change it to rotate the pattern.

Can I use this for any circular pattern?

Yes, this calculator works for any equally spaced circular arrangement, including rivets, spokes, mounting holes, and decorative patterns.

What's the difference between BCD and PCD?

BCD (Bolt Circle Diameter) and PCD (Pitch Circle Diameter) are the same thing—they both refer to the diameter of the circle passing through the bolt centers.

How accurate do my measurements need to be?

For critical applications like flanges under pressure, maintain accuracy to ±0.01 inches or ±0.25mm. For general fabrication, ±0.05 inches is usually acceptable.