How to use this calculator
- Choose solve direction. Pick whether you know torque or target bolt tension.
- Enter diameter and nut factor. Use nominal bolt diameter and a nut factor for the surface/lubrication condition.
- Enter torque or tension. Enter the known torque or desired preload.
- Read the result. Read bolt tension and the corresponding torque in metric or imperial units.
How it works
The short-form nut-factor relation is:
T = K · F · d
Rearranged, bolt tension is F = T/(K·d). The calculator keeps d in
consistent length units internally and reports force and torque in the active
unit system.
K is not a material constant. It is an empirical friction factor for the whole joint: threads, nut face, washer, plating and lubricant. That is why two bolts at the same torque can have very different tension.
Worked example
Verified against the live calculator
For an M10 bolt at 30 N·m with K = 0.20, tension is
30 × 1000 ÷ (0.20 × 10) = 15,000 N. In the reverse direction, a
15 kN target preload on the same bolt needs 0.20 × 15,000 × 10 ÷ 1000,
or 30 N·m.
Frequently asked questions
How do you convert torque to bolt tension?
Use the short-form torque–tension relation F = T/(K·d), where T is torque, K is nut factor and d is nominal bolt diameter. The same equation rearranges to T = K·F·d for target preload.
What nut factor should I use?
Typical rough values are K ≈ 0.20 dry/as-received, 0.15 lubricated and 0.10 with anti-seize, but the real value depends on finish, lubricant, washer, thread condition and reuse.
Is torque a precise way to set preload?
No. Torque control often gives only about ±25–35% preload accuracy because friction dominates the torque. Use direct tensioning, turn-of-nut or measured elongation for critical joints.
How is this different from the bolt preload calculator?
This page is the bidirectional torque-to-tension utility. The bolt preload calculator focuses on clamp force from an applied torque and related joint notes.
Method & assumptions
- Uses nominal diameter, not pitch diameter or tensile stress area.
- Assumes the nut factor K already captures thread and bearing friction.
- Does not check proof load, joint stiffness, embedment, relaxation or fatigue.