Custom Bolt Circle Gasket Calculator

Neighbouring holes sit 360°/N apart on a circle of radius PCD/2. The chord between two points on a circle that subtend an angle α is 2r·sin(α/2), so S = 2·(PCD/2)·sin(180°/N) = PCD·sin(180°/N). Hole k sits at θk = θ0 + k·360°/N, counter-clockwise from +X.

S = PCD × sin(180° / N)
S = 190.50 × sin(180° / 8) = 72.90 mm
Ligament between holes = S − d = 72.90 − 19.05 = 53.85 mm
Xk = PCD/2 × cos(θ0 + k × 360°/N), Yk = PCD/2 × sin(θ0 + k × 360°/N); straddle start θ0 = 180°/N

A few patterns repeat across industries. Four- and eight-hole patterns dominate small pipe flanges, sixteen and more appear from about 12 inches or DN300 up, and three-hole patterns are common on exhaust collectors and water outlets, where an odd count makes measuring the PCD harder.

How the generator uses these numbers

In the studio, hole k is placed at θ0 + k × 360°/N, counter-clockwise from +X with the centre at (0, 0) and Y up — the same convention as this table and the free check-sheet TXT. Straddle on means θ0 = 180°/N, which is what ASME B16.5 and EN 1092-1 drillings use.

The "Open in generator" button carries PCD, N, d and the start angle to the main studio and adds an OD and ID three hole diameters outside and inside the bolt circle. Adjust them to the real flange; the web check then shows how much material is left on each side of the holes.

Common mistakes

• Measuring centre to centre across the flange on an odd hole count — there is no opposite hole, so use the chord.
• Forgetting the straddle offset: with 8 holes the first one sits at 22.5°, not on the horizontal centreline.
• Rounding the PCD to a whole inch on metric flanges — DN100 PN16 is 180 mm, not 7 inches (177.8 mm).
• Using the bolt diameter instead of the hole diameter for the ligament; holes are normally 1–2 mm larger than the bolt.

Even N (4, 6, 8…): two holes sit exactly opposite. Measure from the near edge of one hole to the far edge of the opposite hole; that edge-to-edge distance equals the PCD, because the two half-holes cancel out.

Odd N (3, 5…): no hole is opposite another. Measure the chord S between two neighbouring centres and divide by sin(180°/N): a 3-bolt flange with S = 86.60 mm has PCD 86.60 / 0.86603 = 100.0 mm.

Ligament and edge rules

The ligament S − d is the material between two holes. The studio also checks each hole against the bore and the outer edge and raises THIN WARNING when any web is under 2.5 mm. With 36 holes of Ø19.05 on PCD 190.5 the chord is only 16.60 mm, so the holes overlap and the design is refused.

Checking a cut gasket with calipers

The chord formula generalises to any two holes: holes k steps apart are PCD·sin(k·180°/N) apart, centre to centre. On the 8-hole ASME 4" Class 150 pattern (PCD 190.5 mm) that gives 72.90 mm for neighbours, 134.70 mm for every second hole, 176.00 mm for every third and 190.50 mm straight across. On a 12-hole DN200 PN16 gasket (PCD 295 mm) the same check reads 76.35, 147.50 and 208.60 mm, then 295.00 mm across. Measure two or three of these on the first cut part before running a batch.

The two error types look different. A wrong PCD changes every chord by the same factor: 0.5 mm too large on PCD 190.5 adds only 0.19 mm to the 8-hole chord, so check the across-flange distance instead. A wrong start angle leaves every chord correct but rotates the whole pattern; 1° on PCD 190.5 moves each hole 1.66 mm along the circle, enough to foul a 5/8" bolt in a 3/4" hole.

Use the result in the gasket DXF generator or check your sheet settings in the laser kerf guide.