Stud Bolt Length Calculation for Flanges

Quick Answer

Stud bolt length for a flanged joint equals the sum of both flange thicknesses + gasket thickness + two nut heights + thread protrusion allowance. For standard ASME B16.5 flanges, bolt lengths are pre-calculated in the bolt chart tables. For non-standard joints (e.g., vessel nozzles, special gaskets, spacers), manual calculation is required.

The correct stud bolt length ensures that the bolt threads engage both hex nuts fully while allowing enough protrusion for visual verification after torquing. A bolt that is too short risks insufficient thread engagement; a bolt that is too long wastes material and may interfere with insulation or adjacent components.

Bolt Length Formula

The general formula for stud bolt length (FTF; first thread to first thread) is:

L = F1 + F2 + G + 2N + P

Where:

Symbol	Component	Typical Value
F1	Thickness of main flange (at bolt circle)	Per ASME B16.5 dimensional table
F2	Thickness of companion flange (at bolt circle)	Per ASME B16.5 dimensional table
G	Gasket thickness	1/8” (SWG), 1/16” (soft), 0 (RTJ; ring sits in groove)
N	Nut height (each)	Per ASME B18.2.2 heavy hex nut table
P	Thread protrusion allowance	Typically 1/4” (6 mm) per end = 1/2” total

Nut Height by Bolt Diameter

Bolt Diameter (in)	Heavy Hex Nut Height (in)	Bolt Diameter (in)	Heavy Hex Nut Height (in)
1/2	31/64	1-1/4	1-1/16
5/8	39/64	1-3/8	1-3/16
3/4	47/64	1-1/2	1-5/16
7/8	55/64	1-5/8	1-7/16
1	63/64	1-3/4	1-1/2
1-1/8	1	2	1-3/4

Worked Example

Calculate the stud bolt length for a NPS 4 Class 300 RF flange joint with a spiral wound gasket.

Component	Value	Source
Main flange thickness (F1)	1.50” (38.1 mm)	ASME B16.5, NPS 4 CL300
Companion flange thickness (F2)	1.50” (38.1 mm)	Same
SWG gasket compressed thickness (G)	0.125” (3.2 mm)	Nominal 1/8”
Nut height (N), 3/4” bolt	0.734” (18.6 mm)	ASME B18.2.2
Thread protrusion (P)	0.50” (12.7 mm)	1/4” per end

L = 1.50 + 1.50 + 0.125 + 2(0.734) + 0.50 = 5.09”

Round up to the nearest standard length: 5-1/4” (133 mm).

The ASME B16.5 bolt chart lists 5-1/4” for NPS 4 Class 300 RF with 3/4” diameter bolts, confirming the calculation.

RTJ Flange Adjustment

For RTJ flanged joints, the gasket sits inside the groove, so the effective gasket thickness added to the bolt length is zero. However, RTJ flanges have a contact face that protrudes beyond the flange face. The bolt chart accounts for this; always use the RTJ column in the ASME B16.5 bolt chart when specifying RTJ joints.

Non-Standard Joints

For connections not covered by ASME B16.5 tables (e.g., valve-to-vessel, special spacer rings, insulation joints), calculate the bolt length manually using the formula above and round up to the nearest standard increment (typically 1/4” or 5 mm increments).

Additional components to include when present:

Additional Component	Added to Length
Insulation spacer ring	Ring thickness
Flange isolation kit (gasket + sleeves)	Kit total thickness
Washers (2x)	2 x washer thickness
Spectacle blind / spacer	Blind or spacer thickness

Pro Tip

When ordering stud bolts for an entire project, generate a bolt take-off from the line list and piping class. Group bolts by diameter and length to consolidate purchase orders. Over-ordering by 5-10% is standard practice to cover field adjustments and damaged bolts.

Apply bolt torque per ASME PCC-1 guidelines using a star pattern. After first torque pass, verify that 2-3 full threads protrude beyond each nut.

Read the full guide to gasket selection