Bolt Torque Calculator
Calculate the tightening torque for a bolt based on bolt size, grade, and friction coefficient.
Formula
T = K × F × d; F ≈ 0.85 × tensile strength × tensile area
- K = nut factor; d = bolt diameter; F = bolt preload.
- Grade 5: 120 ksi; Grade 8: 150 ksi; metric 8.8: ~120 ksi.
- Always torque in sequences (50%, 75%, 100%) for critical joints.
1/2 in Grade 5 bolt, dry (K=0.2)
Inputs
- Bolt Diameter: 0.5 in
- Bolt Tensile Strength: 120 ksi
- Nut Factor (K): 0.2
T ≈ 0.2 × 5,500 × 0.5 / 12 ≈ 45.8 ft-lbs.
Frequently asked questions
When should I use lubricated torque?
Use a lower nut factor (around 0.15) when threads are lubricated to avoid over-stressing the bolt for the same clamp load.
What is the nut factor (K)?
It bundles thread and head friction into one number: about 0.2 dry, 0.17 zinc-plated, and 0.15 lubricated.
Why torque in stages?
Tightening in steps (50%, 75%, 100%) and in a cross pattern spreads the load evenly and prevents distortion on critical joints.
Does higher tensile strength mean more torque?
Yes. A stronger bolt can carry more preload, so its recommended torque is higher for the same diameter.