Manufacturing · Fastening
Bolt Torque & Preload Calculator
Convert tightening torque to clamp force for metric fasteners — pick your thread, watch the wrench diagram respond, and see your proof-load margin instantly.
Using nut factor K = 0.2 — Plain, unlubricated steel.
Result
Torque → Preload
F = T / (K · d)
Sweet spot — ~72% of proof
Sitting in the classic 70–80% proof window — a solid target for general structural joints.
Installation torque T
50N·m
Tensile stress σ
431MPa
Percent of proof load %
71.8%
Proof load Fₚ
34.8kN
Factor of safety (proof) FoS
1.39
~75% of proof: 26.1 kN at 52.2 N·m.
Short-form nut-factor method for metric fasteners — first-pass sizing, not certification.
When you tighten a bolt, most of the applied torque is consumed by friction under the head and in the threads; only a fraction becomes axial clamp force (preload). The widely used short-form relationship captures this with a single nut factor K:
where T = tightening torque (N·m), K = nut factor (dimensionless), d = nominal thread diameter (m), and F = bolt preload / clamp force (N). Rearranged: F = T / (K · d).
- Torque from preloadT = K · d · F
- Preload from torqueF = T / (K · d)
- Tensile stressσ = F / A_s
- Proof loadF_proof = A_s · S_proof
- Factor of safetyFoS = F_proof / F
- Typical targetF ≈ 0.75 · F_proof
| Condition | K (typical) | Notes |
|---|---|---|
| Dry / as-received | 0.2 | Plain, unlubricated steel |
| Zinc plated | 0.22 | Electroplated, dry |
| Lightly oiled | 0.18 | Machine oil on threads |
| Molybdenum disulfide | 0.14 | MoS₂ grease / paste |
| PTFE / wax | 0.12 | PTFE coating or wax |
| Class | S_proof (MPa) | R_p0.2 (MPa) | R_m (MPa) |
|---|---|---|---|
| 4.8 | 310 | 340 | 420 |
| 8.8 | 600 | 640 | 800 |
| 10.9 | 830 | 940 | 1040 |
| 12.9 | 970 | 1100 | 1220 |
For an M10 bolt (d = 0.010 m, A_s = 58 mm²) tightened to T = 50 N·m with a dry nut factor K = 0.20: preload F = 50 / (0.20 × 0.010) = 25 000 N = 25 kN. Tensile stress σ = 25 000 / 58 ≈ 431 MPa. The class 8.8 proof load is 58 × 600 = 34.8 kN, so the preload sits at about 72% of proof — a sensible target.
What is the nut factor K?
K is an empirical torque coefficient that lumps together thread friction, under-head friction and thread geometry. Typical values range from about 0.12 (waxed/PTFE) to 0.22 (zinc plated, dry). Validate for critical joints.
Why does lubrication reduce the required torque?
Lubrication lowers friction, so a smaller fraction of torque is lost and more goes into preload. The same torque on a lubricated bolt produces a higher clamp force — match K to the actual thread condition.
What preload should I aim for?
For general structural joints, 70–80% of proof load is a common target. Lower values may be used where fatigue, gasket seating or relaxation are concerns. Follow the joint design specification where one exists.
- The short-form method does not separate thread and under-head friction or account for joint stiffness, embedment or relaxation.
- Nut factors are representative starting points; real K depends on plating batch, surface finish and lubricant.
- Results assume a ductile steel fastener in pure tension — not for safety-critical certification without verification.