# Bolt Circle Calculator

Calculate bolt positions on a circular pattern. Enter bolt circle diameter and number of holes to get X/Y coordinates, chord length, and angular spacing.

## What this calculates

Laying out bolt holes on a circular pattern is a common machining and fabrication task. Enter the bolt circle diameter (BCD), number of bolts, and starting angle to get exact X/Y coordinates for each hole, plus the spacing between them.

## Inputs

- **Bolt Circle Diameter (BCD)** (in) — min 0.1, max 1000 — Diameter of the circle passing through bolt hole centers
- **Number of Bolts** — min 2, max 72
- **Start Angle** (°) — min 0, max 360 — Angle of the first bolt from the top (0 = 12 o'clock)
- **Bolt Hole Diameter** (in) — min 0 — Optional: diameter of each bolt hole

## Outputs

- **Bolt Circle Radius** (in)
- **Angular Spacing** (°)
- **Chord Length (Bolt-to-Bolt)** (in) — Straight-line distance between adjacent bolts
- **Arc Length (Bolt-to-Bolt)** (in) — Distance along the circle between adjacent bolts
- **Bolt Positions (X, Y)** — formatted as text — Coordinates from center

## Details

## How Bolt Circle Layout Works

A bolt circle is defined by its diameter (BCD) and the number of evenly spaced holes around it. The center of each hole sits on the circle, and the angular spacing between holes is simply 360 divided by the number of bolts.

**Key formulas:**
- **Angular spacing** = 360 / number of bolts
- **Chord length** = 2 x radius x sin(180 / number of bolts)
- **X position** = radius x sin(angle)
- **Y position** = radius x cos(angle)

### Common Bolt Circle Patterns

- **4-bolt:** 90 degrees apart, used on smaller flanges and wheel hubs
- **6-bolt:** 60 degrees apart, common on pipe flanges and truck wheels
- **8-bolt:** 45 degrees apart, heavy-duty flanges and large machinery
- **12-bolt:** 30 degrees apart, large industrial flanges

### Measuring an Existing Bolt Circle

If you need to find the BCD of an existing part, measure the center-to-center distance between two adjacent bolts. Then use: BCD = distance / sin(180 / number of bolts).

## Frequently Asked Questions

**Q: What is a bolt circle diameter (BCD)?**

A: The bolt circle diameter (BCD) is the diameter of the imaginary circle that passes through the center of each bolt hole. For example, a 6-inch BCD with 4 bolts means all four bolt hole centers sit on a circle that is 6 inches across.

**Q: How do I find the bolt circle diameter from bolt spacing?**

A: Measure the center-to-center distance between two adjacent bolts. Then divide that distance by sin(180 / number of bolts). For a 4-bolt pattern with 4.243 inches between adjacent bolts: 4.243 / sin(45) = 4.243 / 0.7071 = 6 inch BCD.

**Q: What is the chord length on a bolt circle?**

A: The chord length is the straight-line distance between the centers of two adjacent bolt holes. It equals 2 x radius x sin(180 / n), where n is the number of bolts. This is useful for verifying your layout with calipers or a ruler.

**Q: How do I lay out a bolt circle without a CNC machine?**

A: Use a compass to draw the bolt circle, then step off the chord length with dividers. For precision, calculate X/Y coordinates from center and use a DRO (digital readout) on a mill. You can also use a rotary table set to the angular spacing between holes.

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Source: https://vastcalc.com/calculators/construction/bolt-circle
Category: Construction
Last updated: 2026-04-08