Flange Bolt Torque

Where torque values come from, computed for every bolt diameter in ASME B16.5 — plus the tightening sequence that decides whether the joint actually seals.

Torque equation: T = K · F · d
Nut factor K: 0.16 lubricated · 0.20 dry
Typical target stress: 50,000 psi for A193 B7
Pattern: Cross-pattern, in at least four passes

Short answer. Bolt torque is not a property of the flange — it is whatever produces the bolt stress the joint needs. The working relationship is T = K·F·d, where F is the target bolt load, d is the nominal bolt diameter and K is the nut factor, around 0.16 well lubricated and 0.20 dry. Lubrication is not a detail: a dry bolt reaches roughly 20% less preload at the same torque.

The torque equation

Torque is an indirect way of producing bolt tension, and most of it is spent overcoming friction rather than stretching the bolt. That is why the nut factor dominates the result:

T (lb-ft) = K · F (lbf) · d (in) / 12
F = target bolt stress × tensile stress area
A_s = π/4 · (d − 0.9743/n)²

Roughly half the applied torque is lost to friction under the nut face, and another 40% to friction in the threads. Only about 10% actually goes into stretching the bolt — which is why torque control has a scatter of ±25 to ±30% on bolt load even when done carefully, and why critical joints are bolt-tensioned or ultrasonically measured instead.

Torque by bolt size

Computed for ASTM A193 Grade B7 studs at the target stresses shown. These are the bolt diameters that appear in the ASME B16.5 flange tables.

Computed bolt torque for A193 B7 studs, T = K·F·d.
Bolt size	Thread	Stress area	Bolt load at 50 ksi	Torque, lubricated (lb-ft)	Torque, dry (lb-ft)	Torque at 40 ksi, lubricated (lb-ft)
1/2 in	13 TPI	0.1419 in²	7.1 kip	47	59	38
5/8 in	11 TPI	0.2260 in²	11.3 kip	94	118	75
3/4 in	10 TPI	0.3345 in²	16.7 kip	167	209	134
7/8 in	9 TPI	0.4617 in²	23.1 kip	269	337	215
1 in	8 TPI	0.6057 in²	30.3 kip	404	505	323
1 1/8 in	8 TPI	0.7905 in²	39.5 kip	593	741	474
1 1/4 in	8 TPI	0.9997 in²	50.0 kip	833	1,041	666
1 3/8 in	8 TPI	1.2335 in²	61.7 kip	1,131	1,413	905
1 1/2 in	8 TPI	1.4918 in²	74.6 kip	1,492	1,865	1,193
1 5/8 in	8 TPI	1.7747 in²	88.7 kip	1,923	2,403	1,538
1 3/4 in	8 TPI	2.0822 in²	104.1 kip	2,429	3,036	1,943
1 7/8 in	8 TPI	2.4141 in²	120.7 kip	3,018	3,772	2,414
2 in	8 TPI	2.7706 in²	138.5 kip	3,694	4,618	2,955
2 1/4 in	8 TPI	3.5573 in²	177.9 kip	5,336	6,670	4,269
2 1/2 in	8 TPI	4.4421 in²	222.1 kip	7,404	9,254	5,923
2 3/4 in	8 TPI	5.4251 in²	271.3 kip	9,946	12,433	7,957
3 in	8 TPI	6.5063 in²	325.3 kip	13,013	16,266	10,410
3 1/2 in	8 TPI	8.9632 in²	448.2 kip	20,914	26,143	16,731

Lubricated assumes K = 0.16, dry K = 0.20. These are computed reference values, not a substitute for the gasket manufacturer's assembly procedure or a project-specific bolt-up specification.

For a broader torque reference beyond flange bolting — fastener grades, metric sizes and general assembly torque — see torquespec.org.

Worked example: NPS 6 Class 300

ASME B16.5 gives an NPS 6 Class 300 flange 12 bolts of 3/4 in diameter on a 10.62 in bolt circle.

Stress area per bolt: A_s = 0.3345 in²
Target load per bolt at 50,000 psi: 16.7 kip
Torque, lubricated: T = 0.16 × 16,723 × 0.750 / 12 = 167 lb-ft
Total bolt load on the joint: 12 × 16.7 = 201 kip

That total is the number worth carrying away. The joint is held together by hundreds of thousands of pounds of clamping force, and the gasket only seals while that force stays above what the internal pressure is trying to push apart.

Tightening sequence

Sequence matters as much as the final torque. Tightening one bolt fully before its neighbours cocks the flange and crushes the gasket unevenly — the joint then leaks at low pressure no matter what the torque wrench said.

Flange bolt-up sequence, following ASME PCC-1 practice.
Pass	Torque	Pattern
1 — snug	20–30% of target	Cross-pattern (star), hand tight then snug
2	50–70% of target	Cross-pattern
3	100% of target	Cross-pattern
4 — check	100% of target	Clockwise, bolt to bolt, until no nut turns
5 — retorque	100% of target	After 4–24 hours, or after the first thermal cycle, to recover gasket relaxation

ASME PCC-1 Guidelines for Pressure Boundary Bolted Flange Joint Assembly is the reference for joint assembly and gives legacy and alternative patterns in full.

What changes the number

Gasket type is the largest single influence on target stress. A spiral wound gasket needs enough load to seat the winding but not so much that it crushes past its compression stop; a kammprofile needs less; a sheet gasket less again; and an RTJ ring needs enough to yield it into the groove. Always take the seating stress from the gasket manufacturer and work back to torque from there, rather than starting from a generic bolt torque table.

Common questions

How do I calculate flange bolt torque?

T = K · F · d / 12 for lb-ft, where F is the target bolt load (bolt stress × tensile stress area), d is the nominal diameter in inches, and K is the nut factor — about 0.16 lubricated and 0.20 dry.

What is the nut factor K?

An empirical coefficient rolling up thread and nut-face friction. 0.16 is typical for a well-lubricated stud with anti-seize, 0.20 for a dry one. Because most of the torque goes into friction, K dominates the resulting bolt load.

What bolt stress should I target for A193 B7 studs?

50,000 psi is a common general-purpose target, with 40,000 psi used for softer gaskets and lower classes. B7 has a minimum yield of 105,000 psi, so 50 ksi is under half yield.

Why does a flange joint need retorquing?

Gaskets relax and bolts creep slightly after initial bolt-up, and the first thermal cycle relaxes them further. A retorque after 4 to 24 hours or after the first heat-up recovers the lost load.

Is torque an accurate way to set bolt load?

Not very. Scatter of ±25 to ±30% on the resulting bolt load is normal even with a calibrated wrench, because friction varies. Critical joints use hydraulic tensioning or ultrasonic bolt measurement instead.