⚙️ Bolt Circle Calculator
Calculate bolt circle dimensions and coordinates
Minimum 3 bolts
Bolt circle diameter
Optional: For hole spacing calculations
How to Use This Calculator
Enter Number of Bolts
Input number of bolts (minimum 3). Common values: 4 (square), 6 (hexagon), 8 (octagon). All bolts are evenly spaced around the circle.
Enter Circle Diameter
Input bolt circle diameter in inches. This is the diameter of the circle passing through the center of all bolt holes.
Enter Bolt Diameter (Optional)
Optionally enter bolt diameter to calculate minimum recommended hole spacing. Rule of thumb: minimum spacing = bolt diameter × 2.5.
Calculate and Review
Click "Calculate Bolt Circle" to see angular spacing, chord length, arc length, bolt coordinates (X, Y), and minimum hole spacing. Use coordinates for CNC programming or layout.
Formula
Angular Spacing = 360° / Number of Bolts
Chord Length = 2 × R × sin(angle / 2)
Arc Length = (Angle × π × R) / 180
Bolt Coordinates: X = R × cos(angle), Y = R × sin(angle)
Example 1: 6 bolts, 4" diameter
Step 1: Radius = 4 / 2 = 2.0"
Step 2: Angular Spacing = 360 / 6 = 60°
Step 3: Chord Length = 2 × 2.0 × sin(60°/2) = 4.0 × sin(30°) = 4.0 × 0.5 = 2.0"
Step 4: Arc Length = (60 × π × 2.0) / 180 = 2.094"
Example 2: 8 bolts, 5" diameter
Step 1: Angular Spacing = 360 / 8 = 45°
Step 2: Radius = 5 / 2 = 2.5"
Step 3: Chord Length = 2 × 2.5 × sin(45°/2) = 5.0 × sin(22.5°) = 1.914"
About Bolt Circle Calculator
The Bolt Circle Calculator is an essential tool for machinists, mechanical engineers, and fabricators who need to calculate bolt circle dimensions and coordinates for evenly spaced bolts. This calculator implements trigonometric formulas to determine angular spacing, chord length, arc length, and bolt coordinates for bolt circle layouts.
When to Use This Calculator
- Bolt Circle Design: Calculate dimensions for bolt circle layouts
- CNC Programming: Generate bolt coordinates for CNC machining
- Fabrication Planning: Determine bolt spacing and positions
- Flange Design: Design flanges with evenly spaced bolt holes
- Educational Use: Learn and understand bolt circle calculations
Why Use Our Calculator?
- ✅ Complete Dimensions: Shows angular spacing, chord length, arc length, and coordinates
- ✅ Accurate Formulas: Uses trigonometric equations for precise calculations
- ✅ Bolt Coordinates: Provides X, Y coordinates for each bolt position
- ✅ Hole Spacing: Calculates minimum recommended hole spacing
- ✅ Time Savings: Instant calculations eliminate manual math
Understanding Bolt Circles
Basic Principle: Bolt circle is a circle passing through the center of all bolt holes. All bolts are evenly spaced around the circle. Angular spacing is 360° divided by number of bolts. Chord length is the straight-line distance between adjacent bolts, calculated as 2 × R × sin(angle / 2).
Bolt Coordinates: Bolt positions are calculated using X = R × cos(angle) and Y = R × sin(angle), where angle is measured from horizontal (0°). First bolt is typically at 0°, subsequent bolts are spaced by angular spacing. Coordinates are relative to circle center.
Hole Spacing: Minimum recommended hole spacing is bolt diameter × 2.5 (rule of thumb) to ensure adequate material between holes and prevent weakening. Actual spacing depends on material, load, and application requirements.
Tips for Best Results
- Accurate Diameter: Measure bolt circle diameter accurately (through bolt hole centers)
- Verify Spacing: Check that chord length meets minimum hole spacing requirements
- Use Coordinates: Use bolt coordinates for CNC programming or precise layout
- Consider Material: Account for material strength when determining hole spacing
- Check Standards: Verify bolt spacing meets applicable standards and codes
Frequently Asked Questions
How do I calculate bolt circle dimensions?
Angular Spacing = 360° / Number of Bolts. Chord Length = 2 × R × sin(angle / 2), where R is radius and angle is angular spacing. For example, 6 bolts, 4" diameter: Angular Spacing = 360 / 6 = 60°, Radius = 2", Chord Length = 2 × 2 × sin(30°) = 2.0". The calculator does this automatically.
What is the difference between chord length and arc length?
Chord length is the straight-line distance between adjacent bolt centers. Arc length is the curved distance along the circle between adjacent bolt centers. For small angles, they're similar. For large angles, arc length is longer. Chord length is used for layout, arc length is used for verification.
How do I use bolt coordinates?
Bolt coordinates (X, Y) are relative to circle center. Use these for CNC programming, CAD layout, or manual marking. X and Y are in inches (or your unit). First bolt is typically at (R, 0) or (0, R) depending on starting angle. The calculator shows coordinates for all bolts.
What is minimum hole spacing?
Minimum recommended hole spacing is bolt diameter × 2.5 (rule of thumb). For example, 0.25" bolt requires minimum spacing of 0.625". This ensures adequate material between holes and prevents weakening. Actual spacing depends on material, load, and application. Check applicable standards for specific requirements.
Can I use this for non-circular bolt patterns?
This calculator is for circular bolt patterns (evenly spaced bolts around a circle). For rectangular patterns, linear patterns, or non-circular patterns, different calculations are needed. For irregular patterns, use CAD software or manual calculations for each bolt position.