Flange Bolt Hole Layout Calculator

Generate an equally spaced flange bolt-hole layout with exact hole angles, X-Y drilling coordinates, chord spacing and radial clearances.

Flange layout

Pattern orientation

Layout Results

Bolt circle radius
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Angular spacing
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Adjacent hole spacing
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Bolt circle circumference
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Radial hole clearance
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Outer material margin
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Hole Coordinates

Click Calculate Bolt Hole Layout to generate the drilling table.

Quick Layout Summary

Results will appear here after calculation.

Formula

Bolt circle radius: R = BCD / 2 Angular spacing: A = 360° / N Hole angle: θi = Start Angle + (i × A) Bolt-hole coordinates: X = R × cos(θ) Y = R × sin(θ) Adjacent center-to-center spacing: C = 2R × sin(A / 2) Bolt circle circumference: C = π × BCD Radial clearance from hole center: Cr = R − d / 2 Outer material margin: M = (Flange OD − BCD − d) / 2
This calculator assumes a circular flange and equally spaced bolt holes. Before drilling a pressure-containing flange, verify the applicable flange standard, nominal size, pressure class, hole diameter, bolt count and orientation against the engineering drawing.

Example

Bolt circle diameter
200 mm.
Hole count
8 holes.
Angular spacing
360 ÷ 8 = 45°.
Bolt circle radius
200 ÷ 2 = 100 mm.
Adjacent hole spacing
2 × 100 × sin(22.5°) ≈ 76.54 mm.
First hole at 0°
X = 100 mm, Y = 0 mm.

Frequently Asked Questions

What is a flange bolt-hole layout? It is the dimensional arrangement used to position bolt holes around a flange.

How are equally spaced holes positioned? The full 360° circle is divided by the number of holes. Every hole is then placed at that angular interval.

What are X and Y coordinates? They locate each hole center relative to the center of the flange and are useful for drilling, CNC programming and layout.

Can I rotate the complete bolt pattern? Yes. Change the starting hole angle. The hole spacing remains unchanged.

Does clockwise versus counter-clockwise change the bolt circle? No. It only reverses the order in which the coordinates are listed around the flange.

What is adjacent hole spacing? It is the straight-line center-to-center distance between neighboring bolt holes.

Does bolt-hole diameter affect the coordinates? No. Hole diameter affects clearances, but not the theoretical center coordinates.

Can this be used for CNC drilling? The generated X-Y coordinates can be used as a layout reference, but production CNC programs should be independently checked before machining.

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