Use our free flange bolt torque calculator to instantly find the target tightening torque per bolt for any flanged joint. Select your B7 stud bolt size, number of bolts, lubrication condition and target bolt stress — the tool returns torque in ft-lb and Nm using the ASME PCC-1 Appendix K simplified method, plus a recommended three-pass tightening sequence. Correct bolt torque is critical to gasket sealing: too little and the joint leaks; too much and you risk crushing the gasket or yielding the stud.
The calculator uses the T = K × D × F formula where K is the nut factor (friction), D is the nominal bolt diameter and F is the required bolt load from your target stress and bolt tensile stress area. It covers standard flange sizes from 1/2” to 2” B7 stud bolts used on ball valves, gate valves and other flanged valve assemblies.
Need to size the valve itself first? Use our Valve Cv Calculator to determine the required flow coefficient before confirming the flange bolt torque. For pressure drop across the valve, see our Valve Pressure Drop Calculator.
How to calculate flange bolt torque
This flange bolt torque calculator uses the ASME PCC-1 Appendix K simplified method — the standard short-form relationship for bolted flange joint assembly. The formula is:
T = K × D × F
where T is the target torque per bolt, K is the nut factor, D is the nominal bolt diameter and F is the required bolt load. The bolt load F equals the target bolt stress multiplied by the bolt tensile stress area per ASME B1.1 UNC. A larger bolt or higher target stress means more bolt load and more torque.
Choosing the nut factor
- Dry / uncoated — K ≈ 0.20. Highest torque required for a given preload. More variable.
- Lubricated — K ≈ 0.16. The standard assumption with thread lubricant on studs and nut face.
- PTFE-coated — K ≈ 0.12. Lowest K and most repeatable. Used on stainless and high-alloy fasteners.
The nut factor is not the coefficient of friction — it is an empirical constant. Because torque and preload are highly sensitive to K, a torque figure is meaningless without stating the lubrication condition. A dry joint can require around 50% more torque than a lubricated joint for the same bolt preload.
Tightening sequence
Tighten in a star (criss-cross) pattern over several passes — typically 30%, 70% then 100% of target torque — followed by a final clockwise check pass. This distributes gasket load evenly and avoids localised crushing. For bolts above approximately 1¼” diameter, hydraulic tensioning is often preferred over torquing to achieve uniform preload. The calculator shows all three pass values automatically. For correct flange selection to go with your bolt-up, browse our range of ball valves by size or contact our team for application support.
How to use this calculator
- Select the bolt size — standard B7 stud sizes from 1/2” to 2” UNC.
- Enter the number of bolts — used to calculate the total joint bolt load.
- Choose the lubrication condition — dry, lubricated or PTFE-coated; this sets the nut factor K.
- Select the target bolt stress — expressed as a percentage of B7 yield strength (105 ksi). 50 ksi (48%) is a common starting point for many gasket types.
The result shows torque per bolt in ft-lb and Nm, individual bolt load in lbf, total joint load, the three-pass tightening sequence and the full equation with your values substituted in. Always verify the resulting gasket stress against the gasket manufacturer minimum seating stress and maximum crush limit.
Frequently Asked Questions
How do you calculate flange bolt torque?+
Use T = K × D × F — torque per bolt equals nut factor × bolt diameter × bolt load. Bolt load F is the target bolt stress times the bolt tensile stress area. This is the ASME PCC-1 Appendix K simplified method.
What is the nut factor (K)?+
An empirically derived constant capturing thread and nut-face friction — roughly 0.20 dry, 0.16 lubricated, 0.12 PTFE-coated. It is not the coefficient of friction, and a small change in K causes a large change in bolt load, which is why lubrication must always be stated.
What torque for a 6 inch Class 150 flange?+
A 6” Class 150 flange uses eight 3/4” B7 studs. Depending on target stress and lubrication, target torque is roughly 130–200 ft-lb per bolt. Confirm against your gasket manufacturer data rather than a generic number.
Should flange bolts be lubricated?+
Almost always — lubricant on the threads and nut face lowers and stabilises the nut factor, giving a more predictable preload. A dry joint can need around 50% more torque than a lubricated one for the same preload.
In what order do you tighten flange bolts?+
In a star (criss-cross) pattern over several passes — commonly 30%, 70% and 100% of target — then a final clockwise pass. ASME PCC-1 details the assembly patterns.
Method: T = K·D·F per ASME PCC-1 Appendix K (simplified). Bolt stress areas per ASME B1.1 / UNC. Reference only — confirm gasket seating stress, bolt grade and final torque against your gasket manufacturer and project specification.
