Bolt Circle Calculator
Calculate the angle and straight-line chord spacing between bolts evenly arranged on a circular flange or pattern.
How this calculator works
A bolt circle pattern places a set number of bolts evenly around a circle of a given diameter, common on flanges, wheels, and pipe fittings. This calculator takes the bolt circle diameter and number of bolts and gives you two numbers: the angle between adjacent bolts, and the straight-line chord spacing between them.
The angle comes from dividing a full circle by the number of bolts. The chord spacing then converts that angle into an actual straight-line distance across the circle, using the sine of half that angle.
Formula: Angle between adjacent bolts = 360° ÷ number of bolts. Chord spacing (straight-line distance between two adjacent bolts) = bolt circle diameter × sin(180° ÷ number of bolts).
Worked example
An 8-inch bolt circle diameter with 6 bolts:
- Bolt circle diameter: 8 in
- Number of bolts: 6
- Angle between bolts: 360 ÷ 6 = 60°
- Chord spacing: 8 × sin(30°) = 4.00 in
Notes
The chord spacing, not the arc length, is the number you’ll actually use with a ruler or calipers. Since it’s a straight-line distance between two adjacent bolt centers, it’s the one measurement you can physically check by placing a tool directly across the pattern, unlike the arc, which follows the curve and can’t be measured with a straight edge.
This math applies to any evenly-spaced circular bolt pattern, regardless of what it’s on. Flanges, wheel lug patterns, and pipe fittings all follow the same relationship between bolt count, circle diameter, angle, and chord spacing, as long as the bolts are spaced equally around the full 360° of the circle.
How to use
Enter the bolt circle diameter and the number of bolts in the pattern. The calculator returns the angle between adjacent bolts and the chord spacing, which you can use to lay out a new pattern or verify an existing one with a straightedge or calipers.
This is a planning estimate using standard mechanical engineering formulas, not a substitute for engineering design. Confirm actual fastener sizing and torque specs with the fastener manufacturer or a structural engineer.
Frequently asked questions
What is a bolt circle diameter?
The bolt circle diameter is the diameter of the imaginary circle that passes through the center of every bolt hole in an evenly spaced pattern. It's the reference measurement used to describe flanges, wheel lug patterns, and pipe fittings, since it defines exactly where each hole sits relative to the others.
Why does the angle between bolts matter?
The angle tells you how far apart the bolts are positioned around the full circle, based on how many bolts are in the pattern. It's the number you'd use with a protractor or CAD software when laying out the pattern from the center point, before converting it into a straight-line measurement you can check with a tool.
What is chord spacing and why not just measure the arc?
Chord spacing is the straight-line distance between two adjacent bolts, measured directly across rather than along the curve of the circle. It's what a ruler or calipers actually measures in the real world, since you can't lay a straight tool along a curved arc. That makes chord spacing the practical number for layout and verification.
Does this work for any number of bolts?
Yes, as long as the bolts are evenly spaced around the full circle. This applies whether you're working with a 4-bolt flange, a 6-lug wheel pattern, or a 12-bolt pipe fitting. The math is the same in every case — only the bolt circle diameter and bolt count change.
Estimates only. Verify quantities with your supplier before purchasing.