Pipe Flange Bolt Circle Calculator

Calculate bolt circle diameter, equally spaced bolt-hole positions, angular spacing and chord spacing for pipe flange layout.

Flange dimensions

Bolt circle diameter

Number of bolt holes

Bolt-hole diameter

Flange outside diameter

Dimension unit

Bolt-hole layout

First hole angle

Layout rotation

Angles are measured from the positive X-axis through the flange center. Hole centers are equally spaced around the bolt circle.

Calculated Results

Bolt circle radius

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Angular spacing

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Bolt-circle circumference

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Adjacent hole spacing

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Hole edge clearance

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Hole-to-hole minimum center distance

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Bolt-Hole Coordinates

Calculate to generate the bolt-hole layout.

Formula

Bolt circle radius: R = BCD / 2 Angular spacing: θ = 360° / N Bolt-hole coordinates: x = R × cos(θ) y = R × sin(θ) For hole i: θi = Start Angle + i × angular spacing Adjacent hole chord spacing: C = 2R × sin(θ / 2) Bolt circle circumference: Ccircle = π × BCD Radial hole edge clearance: E = (BCD − hole diameter) / 2 Approximate outside radial margin: M = (Flange OD − BCD − hole diameter) / 2

This calculator assumes equally spaced holes on a circular bolt pattern. Always verify the applicable flange standard, pressure class, nominal pipe size, hole size and orientation before manufacturing or drilling a pressure-containing flange.

Example

Bolt circle

200 mm diameter.

Number of holes

8 equally spaced holes.

Angular spacing

360° ÷ 8 = 45°.

Adjacent hole center spacing

2 × 100 × sin(22.5°) ≈ 76.54 mm.

Hole coordinates

The calculator generates the X/Y center coordinates for every hole from the flange center.

Frequently Asked Questions

What is a bolt circle diameter? It is the diameter of the imaginary circle passing through the centers of all equally spaced flange bolt holes.

How is angular spacing calculated? For equally spaced holes, divide 360° by the number of holes.

How is the distance between adjacent holes calculated? It is the chord of the bolt circle: 2R × sin(θ/2).

Can I calculate bolt-hole coordinates? Yes. The calculator produces X and Y coordinates measured from the flange center.

Does hole diameter affect the bolt circle? No. Hole diameter does not change the theoretical bolt circle diameter, but it affects edge clearances and minimum material between holes.

What is radial hole clearance? It is the radial distance from the bolt-hole centerline to the edge of the bolt hole, calculated as BCD/2 minus half the hole diameter.

Can this be used for flange drilling? Yes, for preliminary geometric layout. Production flange drilling should be checked against the applicable flange standard and drawing.

Does the first-hole angle matter? Yes. Rotating the starting angle rotates the entire bolt pattern without changing the bolt circle diameter or spacing.