OFFSHORE TOOLKIT · CALCULATOR 09

Bolt Torque / Flange Tensioning Calculator

Torque required for a target bolt preload using the standard K × D × F relationship, plus a star-pattern bolt-up sequence and multi-pass tightening reference for flange joints.

Metric

Imperial

Bolt

Nominal bolt diameter (mm)

Nut factor (K)

K factors vary significantly with actual lubricant, plating, and thread/surface condition. These are typical starting points only — for a critical joint, get K from an actual torque-tension test per ASME PCC-1.

Target preload

Target bolt preload (kN)

Enter your target preload directly, or use the fields below to calculate it from bolt stress area and target stress — filling both overrides the value above.

Tensile stress area (mm²) — optional

Target bolt stress (MPa) — optional

Common guidance targets roughly 50–75% of the bolt's proof/yield stress — check your applicable spec or ASME PCC-1 for the actual target used on this joint.

REQUIRED TORQUE

—

N·m

Preload used—

K factor—

Diameter—

Number of bolts

Numbers show tightening order — each pass follows this same sequence. This is a general-purpose star/cross pattern; always follow your flange manufacturer's specific documented procedure where one exists.

Pass	Target torque	Pattern
1	~30% of final torque	Star pattern above
2	~60% of final torque	Star pattern above
3	100% of final torque	Star pattern above
4 (final check)	100% of final torque	Full rotational pass (bolt 1 → around to last, in order)

A common multi-pass approach: tighten in the star pattern at increasing percentages of final torque, then finish with one full rotational pass in sequence to confirm every bolt holds final torque. Percentages and pass count vary by spec — this is a widely used general convention, not a universal rule.

Quick reference

Common ISO metric tensile stress areas

Size	Stress area (mm²)
M12	84.3
M16	157
M20	245
M24	353
M27	459
M30	561
M33	694
M36	817

Coarse thread, per ISO 898-1. Always confirm against the actual bolt spec for critical joints.

Typical proof stress by grade

ISO 8.8: ≈ 660 MPa
ISO 10.9: ≈ 940 MPa
ASTM A193 B7: ≈ 720–725 MPa (≈105 ksi)

Illustrative only — always confirm the actual certified proof/yield stress for the specific bolt grade and size in use.

Good practice

Lubricate consistently — a change in lubricant changes K, and therefore the torque needed for the same preload
Torque wrenches should be calibrated and appropriate for the target range
Re-torquing after initial relaxation ("gasket creep") is often required — follow the joint's specific procedure
For high-consequence joints, consider tensioning (hydraulic bolt tensioners) instead of torque alone — torque-tension has real scatter even with a good K factor

Why torque is an imperfect proxy for tension

Most of the applied torque (often 80–90%) overcomes friction under the nut/bolt head and in the threads, not stretching the bolt
Because of this, torque-tension relationships can have real scatter — ±25% variation in achieved preload for the same torque is not unusual with an assumed (not measured) K factor
Where consistent, accurate preload matters most, direct tension measurement or hydraulic tensioning avoids this uncertainty