liminfoFlange Bolt Torque Calculator
Free web tool: Flange Bolt Torque Calculator
Bolt Details
Tightening Sequence (Star Pattern)
Tighten in multiple passes: 30% → 60% → 100% of target torque
About Flange Bolt Torque Calculator
The Flange Bolt Torque Calculator determines the correct tightening torque for bolted pipe flange connections based on ASME flange size, pressure class, gasket type, bolt friction factor (K), and target preload percentage of bolt proof stress. It covers standard flange sizes from 2" (50 mm) to 24" (600 mm) with the corresponding bolt sizes (M16 to M33), bolt counts, and bolt circle diameters pre-loaded for ASME B16.5 flanges.
Mechanical engineers, pipefitters, maintenance technicians, and plant operations personnel use this tool to ensure leak-free flange joints. Correct bolt torque is critical in pressure piping systems: under-tightened bolts allow gasket leakage while over-tightened bolts can crush soft gaskets, stretch bolts beyond their yield point, or warp flange faces. The calculator uses the standard torque-tension formula T = K × d × F, where K is the nut factor, d is the bolt diameter, and F is the target bolt clamping force.
The tool also generates the star-pattern tightening sequence for each flange size — the bolt positions are numbered and ordered so that diametrically opposite bolts are tightened alternately. This distributes gasket stress evenly and prevents the flange from cocking. The recommended tightening procedure (30% → 60% → 100% of target torque in multiple passes) is displayed as a reminder, consistent with ASME PCC-1 guidelines for pressure boundary joint assembly.
Key Features
- Pre-loaded ASME flange data for 12 standard sizes from 2" (50 mm) to 24" (600 mm)
- Calculates torque per bolt in both N·m and ft·lb for immediate use with torque wrenches
- Supports 5 ASME pressure classes: 150, 300, 600, 900, and 1500
- Four gasket types with accurate ASME m and y factors: flat rubber, spiral wound, ring joint metal, and PTFE envelope
- Adjustable friction factor K (nut factor) for dry, lubricated, or anti-seize bolt conditions
- Variable preload percentage of bolt proof stress (Grade 8.8, 580 MPa proof stress)
- Automatically generates star-pattern bolt tightening sequence for uniform gasket loading
- Displays per-bolt and total bolt load (kN) alongside bolt stress area and bolt circle diameter
Frequently Asked Questions
What is the torque-tension formula used in flange bolt calculations?
The standard formula is T = K × d × F, where T is the tightening torque (N·m), K is the nut factor (friction-dependent coefficient, typically 0.10–0.20), d is the nominal bolt diameter (m), and F is the target bolt clamping force (N). The clamping force is calculated as the bolt stress area multiplied by the proof stress of the bolt grade multiplied by the preload percentage.
What is a nut factor (K) and what value should I use?
The nut factor K accounts for thread and bearing surface friction and is the most significant source of torque-tension uncertainty. Typical values: K ≈ 0.20 for as-received (unlubricated) steel bolts; K ≈ 0.15 for lightly oiled bolts; K ≈ 0.10–0.12 for bolts with molybdenum disulfide (anti-seize) lubricant; K ≈ 0.08 for PTFE-tape-lubricated bolts. Always use the K value corresponding to your actual bolt condition.
What is the star (cross) pattern for bolt tightening?
The star pattern requires tightening bolts in diametrically opposite pairs rather than in a circle. For a 4-bolt flange: bolt 1 → bolt 3 → bolt 2 → bolt 4. This approach distributes the gasket seating load evenly across the flange face and prevents the flange from tilting or cocking during assembly, which would result in uneven gasket stress and potential leaks.
Why should bolts be tightened in multiple passes?
Tightening in multiple passes (typically 30% → 60% → 100% of target torque) allows the gasket to seat progressively and allows bolt elongation to equilibrate. After tightening the last bolt in a pass, the first bolt may have lost some tension due to gasket relaxation and elastic interaction. Multiple passes ensure all bolts reach the target preload uniformly, as recommended by ASME PCC-1.
What gasket types are supported and how do the m and y factors affect torque?
The calculator supports flat rubber (m=1.0, y=1.4 MPa), spiral wound flexible graphite (m=3.0, y=69 MPa), ring joint metal (m=6.5, y=179 MPa), and PTFE envelope (m=2.0, y=11 MPa). The m and y factors from ASME Section VIII Appendix 2 define the seating stress required to achieve a seal. Harder gaskets (ring joint) require higher bolt loads to achieve seating, but the calculator uses preload percentage of proof stress as the primary driver of bolt tension rather than gasket seating stress directly.
What bolt grade does this calculator assume?
The calculator uses Grade 8.8 metric bolts with a proof stress of 580 MPa. This is a common standard in industrial piping and mechanical applications. If your application uses different bolt grades (A193 B7, Grade 10.9, etc.), the proof stress will differ and you should adjust the preload percentage accordingly or consult the appropriate bolt specification.
What is ASME pressure class and how does it relate to flange bolt torque?
ASME pressure class (150, 300, 600, 900, 1500) designates the maximum allowable working pressure at a reference temperature. Higher pressure classes require thicker flanges, larger bolts, more bolts, and higher bolt torques to maintain the joint integrity under greater internal pressure and gasket seating requirements. The calculator currently uses the same bolt data per flange size regardless of class — use the pressure class as a reference for system design rather than a direct torque modifier in this tool.
Can I use this calculator for non-ASME flanges (DIN, JIS, etc.)?
The pre-loaded flange data follows ASME B16.5 bolt sizes and bolt circle diameters. DIN (PN6 to PN400) and JIS flanges use slightly different bolt patterns and sizes. However, if you know the bolt diameter, bolt count, and bolt circle diameter for your DIN or JIS flange, you can select the ASME flange with matching bolt size to get an approximate torque calculation, then verify against your specific flange standard.