What torque should I use on a 3/4" B7 stud?

With well-lubricated threads (K = 0.16) at 50% of B7 yield, a 3/4" stud takes about 211 ft-lb. Dry, it's roughly 264 ft-lb. Always follow the project torque table when one exists — these numbers are a starting point.

Why not use yield strength as the target preload?

Bolts must elongate elastically without yielding so they can be retorqued and so gasket relaxation doesn't lose the seal. ASME PCC-1 uses target stresses in the 35–60% of yield range depending on flange class, gasket type, and service. 50% of yield is the field default for B7.

What does the K-factor (nut factor) represent?

K rolls thread friction, under-head friction, and geometry into one number that converts torque into clamping force. Real K varies ±25% in the field, which is why torque-based tightening is less precise than bolt-stretch or hydraulic tensioning. Always lubricate to a known K when the torque table assumes it.

Should flange bolts be torqued in a star pattern?

Yes — ASME PCC-1 specifies a cross (star) pattern in passes. Pass 1 at 30% of final torque, Pass 2 at 60%, Pass 3 at 100%, then at least one rotational pass at 100% going around the flange. This evenly compresses the gasket and prevents one side from sealing before the other.

B7 vs B8M — what's the difference?

B7 is alloy steel — high strength, used for the vast majority of pressure flange bolting. B8M is 316 stainless — used for corrosive service or temperatures below -50°F. B8M Class 1 (annealed) has only ~30 ksi yield, so torque values are about 30% of B7. B8M Class 2 (strain-hardened) is closer to B7 but only available up to ~1".

Flange Bolt Torque Calculator

Per-bolt torque for flange bolting based on bolt diameter, material, lubricant, and target preload. Outputs ft-lb, N·m, and the resulting clamping force.

Select pipe size and flange class — bolt specs auto-fill from ASME B16.5. Covers NPS ½" through 24". For larger sizes use Custom mode.
Defaults: A193 B7, lubricated (K = 0.15), 45% of yield — calibrated to match standard field torque charts.

Pipe size (NPS)

Flange class

Bolt material

Lubrication

Per-bolt torque

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— N·m

Bolt count

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bolts

Bolt diameter

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inches

Total joint load

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How torque-based bolt tightening works

You can't measure clamping force directly with a torque wrench — you measure the torque it takes to get to that force. The relationship is wrapped up in the bolt torque formula:

T = K × F × D

T — applied torque (in-lb; divide by 12 for ft-lb)
K — nut factor, lumps thread friction, under-head friction, and geometry into one number
F — target clamp load (lb)
D — nominal bolt diameter (in)

Target clamp load comes from the bolt's tensile stress area times a percentage of yield. ASME PCC-1 uses 50% of yield as a starting point for most flange bolting — it gives you room for re-torque after gasket relaxation without yielding the stud.

K-factor cheat sheet

0.20 — Dry / as-received. Default if the threads are clean and unlubed. Lots of scatter.
0.16 — Light oil / heavy-bodied grease. ASME PCC-1 default lubricated value.
0.13 — Anti-seize (Never-Seez, Bostik Never-Seez). Good for stainless, high temp, and re-bolting.
0.10 — Moly-based / molybdenum disulfide grease. Lowest practical K — used on critical bolting and tensioner-installed studs.

K can vary ±25% in the field even when everything is "the same." That's the inherent uncertainty in torque-based bolting. For critical flange joints, hydraulic tensioning or bolt-stretch measurement is more precise.

The torquing pattern (ASME PCC-1)

Hand-tight — bring all bolts snug, finger-tight plus a wrench check.
Pass 1: 30% of final torque, crossing star pattern.
Pass 2: 60%, same pattern.
Pass 3: 100%, same pattern.
Pass 4: 100% rotational — clockwise around the flange, bolt to bolt, at least one full lap with no movement.

Mark each bolt with a paint dot after the rotational pass. If you find a bolt that turns, restart from Pass 3.

Lubricated B7 stud torque chart (50% of yield, K = 0.16)

Quick-reference values for the most common flange bolting setup. Values rounded to 5 ft-lb.

Size	Stress area (in²)	Clamp load (lb)	Torque (ft-lb)
1/2"	0.142	7,450	50
5/8"	0.226	11,870	100
3/4"	0.334	17,550	175
7/8"	0.462	24,260	285
1"	0.606	31,820	425
1-1/8"	0.790	41,480	620
1-1/4"	1.000	52,500	875
1-3/8"	1.233	64,730	1,185
1-1/2"	1.492	78,330	1,565
1-5/8"	1.781	93,500	2,025
1-3/4"	2.077	109,000	2,540
2"	2.770	145,400	3,875

B7 yield = 105 ksi for sizes ≤ 2-1/2". K = 0.16 (well-lubricated threads and seating face). Calc this above for other materials and conditions.

Frequently asked questions

What is the bolt torque formula?

T = K × F × D. K is the nut factor (0.20 dry, 0.16 lubricated, 0.13 anti-seize, 0.10 moly). F is target preload. D is nominal bolt diameter. Result in in-lb — divide by 12 for ft-lb.

What torque for a 3/4" B7 stud, lubricated?

About 175 ft-lb at 50% of B7 yield. Run the calculator above for other preload targets.

Why 50% of yield, not 100%?

So the bolt stays elastic. Yielding the stud means you can't re-torque, and gasket relaxation will lose clamp load over time.

What does the K-factor really represent?

All the friction in the bolt-up — thread friction, under-head friction, geometry — rolled into one. Real K varies ±25% in the field even with lubrication.

Star pattern — is it actually necessary?

Yes. Tightening one bolt fully before its neighbor crushes the gasket on one side and lets the other side leak. ASME PCC-1 requires multiple passes in a cross pattern.

B7 vs B8M?

B7 is high-strength alloy steel — the default for pressure flange bolting. B8M is 316 stainless for corrosive/cryogenic service. B8M Cl 1 (annealed) is only ~30 ksi yield, so torque values are roughly 30% of B7.