Bolt Circle Diameter Calculator

Calculation method

Calculate bolt circle diameter from

Adjacent hole center-to-center spacing Number of equally spaced holes

Dimension unit

Calculated Results

Bolt circle diameter

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Bolt circle radius

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Hole count

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Angular spacing

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Adjacent chord spacing

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Bolt circle circumference

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Calculation Details

Enter the measurement data and calculate.

Formula

For equally spaced holes: Adjacent chord: C = 2R × sin(180° / N) Therefore: BCD = C / sin(180° / N) where: BCD = bolt circle diameter C = adjacent hole center-to-center spacing N = number of equally spaced holes R = bolt circle radius From a known radius: BCD = 2R From a chord and included angle: BCD = C / sin(θ / 2) From hole coordinates: R = √(X² + Y²) BCD = 2√(X² + Y²) From two adjacent hole coordinates: C = √[(X2−X1)² + (Y2−Y1)²] Then: BCD = C / sin(180° / N)

For an actual flange, confirm that the measured holes are equally spaced and that the selected holes are adjacent in the bolt pattern. Production flange dimensions should be checked against the applicable standard or engineering drawing.

Example

Adjacent hole spacing

76.536 mm.

Number of holes

8 equally spaced holes.

Half angular spacing

180° ÷ 8 = 22.5°.

Bolt circle diameter

76.536 ÷ sin(22.5°) ≈ 200 mm.

Bolt circle radius

100 mm.

Frequently Asked Questions

What is bolt circle diameter? Bolt circle diameter, or BCD, is the diameter of the imaginary circle passing through the centers of the bolt holes.

How do I calculate BCD from bolt-hole spacing? For equally spaced holes, divide the adjacent center-to-center spacing by sin(180° ÷ number of holes).

What if I know the bolt circle radius? Simply multiply the radius by 2.

Can I calculate BCD from one hole's X-Y coordinates? Yes, provided the coordinates are measured from the bolt-circle center. BCD = 2 × √(X² + Y²).

Can I calculate BCD from two measured holes? Yes, if the holes are adjacent and the total number of equally spaced holes is known.

Does bolt-hole diameter affect BCD? No. BCD passes through the hole centers, so hole diameter does not change the theoretical BCD.

What is the difference between BCD and flange outside diameter? BCD is the diameter through the hole centers. Flange outside diameter is the total outside diameter of the flange.

Can this be used for flange inspection? Yes, as a geometric calculation aid. For acceptance or certification, compare measurements with the applicable flange standard and drawing tolerances.

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