How to Calculate Flange Bolt Length

Quick Answer

Flange bolt (stud bolt) length is calculated by adding: two flange thicknesses + gasket thickness + two nut heights + thread protrusion allowance. The result is rounded up to the nearest standard stud bolt length per ASME B16.5. For raised face flanges with spiral wound gaskets, the formula is: L = 2C + G + 2N + 6mm, where C is flange thickness at bolt circle, G is gasket thickness, and N is nut height.

Correct stud bolt length ensures adequate thread engagement in both nuts, proper gasket compression, and sufficient thread protrusion beyond the nuts after tightening. Bolts that are too short risk insufficient thread engagement; bolts that are too long add unnecessary weight and cost.

Bolt Length Formula

L = 2C + G + 2N + P

Symbol	Parameter	Notes
L	Stud bolt length (cut length)	Rounded up to nearest standard increment
C	Flange thickness at bolt hole	From ASME B16.5 dimensional tables
G	Gasket thickness (compressed)	Spiral wound: ~4.5 mm; Ring joint: 0 (seated in groove)
N	Nut height (thickness)	Per ASME B18.2.2 or project specification
P	Thread protrusion allowance	Typically 3-6 mm per end (1-2 threads minimum)

For ring joint flanges, the gasket sits inside the groove and does not add to the bolt length. The flange thickness at the bolt circle is measured from the raised face or RTJ groove face to the back of the flange.

Nut Heights (ASTM A194 Gr. 2H)

Stud Bolt Diameter (in)	Stud Bolt Diameter (mm)	Nut Height (mm)
1/2	M14	12.7
5/8	M16	15.4
3/4	M20	18.3
7/8	M22	21.4
1	M24	24.2
1-1/8	M27	27.2
1-1/4	M30	30.5
1-3/8	M33	33.3
1-1/2	M36	36.4

Worked Example

Calculate stud bolt length for NPS 6 Class 150 WN flanges with spiral wound gasket:

Parameter	Value	Source
Flange thickness at bolt circle (C)	25.4 mm	ASME B16.5
Gasket thickness (G)	4.5 mm	Spiral wound per ASME B16.20
Stud bolt diameter	3/4 in (M20)	ASME B16.5
Nut height (N)	18.3 mm	ASTM A194
Thread protrusion (P)	6 mm (3 mm per end)	Standard practice

L = 2(25.4) + 4.5 + 2(18.3) + 6 = 97.9 mm

Rounded up to standard length: 100 mm (4 inches)

Standard Stud Bolt Length Increments

Stud bolts are available in standard length increments:

Range	Increment
Up to 6 inches (150 mm)	1/4 inch (6 mm)
6 to 24 inches (150-600 mm)	1/2 inch (12 mm)
Above 24 inches (600 mm)	1 inch (25 mm)

Always round up to the next standard increment. For stud bolt material specifications and detailed length tables, see the reference guide.

Common Mistake

Using the flange thickness from the face to the hub (overall thickness) instead of the thickness at the bolt circle. The bolt hole passes through the flange rim, not the hub. Always use the flange thickness at the bolt circle diameter from the ASME B16.5 dimensional tables.

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