# Bolt Circle Calculator

URL: https://speedycalculator.com/calculators/construction/bolt-circle/
Last updated: 2026-10-25

> The bolt circle calculator finds the X and Y coordinates of up to eight equally spaced holes on a bolt circle from the number of holes, circle diameter, start angle and centre, plus the hole-to-hole spacing.

## Inputs

- Number of holes
- Bolt circle diameter (mm, in, cm)
- Angle of first hole (°, rad)
- Centre X (mm, in, cm)
- Centre Y (mm, in, cm)

## Results

- Distance between adjacent holes
- Angle between holes
- Hole 1 – X
- Hole 1 – Y
- Hole 2 – X
- Hole 2 – Y
- Hole 3 – X
- Hole 3 – Y
- Hole 4 – X
- Hole 4 – Y
- Hole 5 – X
- Hole 5 – Y
- Hole 6 – X
- Hole 6 – Y
- Hole 7 – X
- Hole 7 – Y
- Hole 8 – X
- Hole 8 – Y

## Formula

```
Angle of hole k = start angle + (k − 1) × 360° ÷ n
X = Xc + D/2 × cos(angle); Y = Yc + D/2 × sin(angle)
Chord spacing = D × sin(180° ÷ n)
```

## Explanation

Flanges, wheels, motor mounts and pipe fittings use bolt holes evenly spaced around a circle. On a mill or drill press you need the coordinates of each hole relative to the centre. The first hole can start at any angle – at 0° (3 o'clock) or rotated so holes straddle the centreline.

### How to use the Bolt Circle Calculator

- **Number of holes**
- **Bolt circle diameter**
- **Angle of first hole**
- **Centre X**
- **Centre Y**

Results update as you type, and every field accepts the units shown in its menu.

### How it works

Enter the number of holes, the bolt circle diameter, the angle of the first hole and the centre coordinates.

### Tips for an accurate estimate

- Measure angles with a digital angle finder or sliding bevel, not by eye.
- Make a test cut in scrap before cutting expensive stock.
- Small angle errors add up over many joints – check the first assembly before cutting the rest.
- Keep a written list of the dimensions and angles you set on the saw or machine.

### Common mistakes to avoid

- Reading the saw scale as the angle from the edge rather than from square.
- Rounding intermediate results too early.
- Mixing degrees and percent grade.

### Inputs explained

FieldWhat to enterNumber of holesA numberBolt circle diameterUnits: mm, in, cmAngle of first holeUnits: °, radCentre XUnits: mm, in, cmCentre YUnits: mm, in, cm

### What each result means

- **Distance between adjacent holes** = [Bolt circle diameter]×sin(rad(180÷[Number of holes]))
- **Angle between holes** = 360÷[Number of holes]

### Example values

How the main result changes when **Number of holes** changes and everything else stays at the example values above:

Number of holesDistance between adjacent holes (mm)386.6034.564.2796507.540.674934.2021225.882

## Worked example

Inputs — Number of holes: 6; Bolt circle diameter: 100 mm; Angle of first hole: 0 °; Centre X: 0 mm; Centre Y: 0 mm.

Results — **Distance between adjacent holes = 50 mm**; Angle between holes = 60 °; Hole 1 – X = 50 mm; Hole 1 – Y = 0 mm; Hole 2 – X = 25 mm; Hole 2 – Y = 43.301 mm; Hole 3 – X = -25 mm; Hole 3 – Y = 43.301 mm; Hole 4 – X = -50 mm; Hole 4 – Y = 0 mm; Hole 5 – X = -25 mm; Hole 5 – Y = -43.301 mm; Hole 6 – X = 25 mm; Hole 6 – Y = -43.301 mm.

## Key terms

- **PCD**: Pitch circle diameter (bolt circle diameter).

## FAQ

**How do I calculate bolt hole positions?**
X = R cos θ and Y = R sin θ for each hole angle θ, added to the centre coordinates.

**What is the distance between bolt holes?**
D × sin(180°/n): for 6 holes on a 100 mm circle, 50 mm.

**What does PCD mean?**
Pitch circle diameter – another name for the bolt circle diameter.

**What formula does the Bolt Circle Calculator use?**
Angle of hole k = start angle + (k − 1) × 360° ÷ n.