Flange Bolt Torque Calculator
Proper bolt torque is critical to achieving a leak-free flanged joint. Under-torquing leaves gasket contact stress too low; over-torquing can damage the gasket or yield the bolts. This calculator uses the standard torque-tension relationship T = K * F * d, where T is the tightening torque, K is the nut factor (friction coefficient), F is the required bolt preload force, and d is the nominal bolt diameter. Enter the bolt details, number of bolts, and required bolt load to calculate the target torque per bolt.
Bolt torque formula
T = K × F × d
F per bolt = Total bolt load / Number of bolts
Where: T = tightening torque per bolt (N.m), K = nut factor (dimensionless), F = required preload force per bolt (N), d = nominal bolt diameter (m).
The total required bolt load comes from ASME Appendix 2 flange design: the greater of the gasket seating condition (Wm2) and the operating condition (Wm1 = hydrostatic end force + residual gasket load).
Tightening sequence
- Pass 1: Snug all bolts finger-tight to seat the gasket evenly.
- Pass 2: Tighten in cross pattern to 30% of target torque.
- Pass 3: Tighten in cross pattern to 60% of target torque.
- Pass 4: Tighten in cross pattern to 100% of target torque.
- Final pass: Verify all bolts at 100% torque, going around the full circle without cross-pattern. Repeat if any bolt moves.
Flange bolt torque calculator: frequently asked questions
How is flange bolt torque calculated?
The required bolt load is determined by the greater of gasket seating load (Wm2 = 3.14 * b * G * y) and hydrostatic end force plus gasket load (Wm1 = H + Hp). Torque per bolt = (Required bolt load per bolt * K * d), where K is the nut factor (typically 0.15 to 0.20) and d is the bolt nominal diameter.
What is the nut factor K?
The nut factor K (also called the torque coefficient) accounts for friction between the bolt threads and the nut. K ranges from 0.10 for well-lubricated bolts (molybdenum disulfide paste) to 0.20 for dry, unlubricated bolts, and up to 0.30 for corroded or rusty threads. ASME PCC-1 recommends verifying K with a calibrated torque wrench.
What is the gasket seating width b?
The basic gasket seating width b depends on the gasket contact width N. For N up to 12.7 mm, b = N/2. For N greater than 12.7 mm, b = 2.53 * sqrt(N/2). ASME code provides tables of N for standard gasket types and flange facings.
Should I torque bolts in a star pattern?
Yes. ASME PCC-1 recommends tightening bolts in a cross or star pattern in at least four passes: first to 30% of target torque, then 60%, then 100%, then a final verification pass at 100%. This ensures even gasket compression and prevents leaks.
What units should I use for this calculator?
Enter bolt diameter in millimetres (mm), design pressure in MPa, and gasket dimensions in mm. The calculator returns required torque in Newton-metres (N.m). Always verify with the flange specification and gasket manufacturer's recommendations.
Official sources
- ASME PCC-1-2019 Guidelines for Pressure Boundary Bolted Flange Joint Assembly: ASME PCC-1.
- ASME B16.5-2017 Pipe Flanges and Flanged Fittings: ASME B16.5.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.