Quick Answer: How Do You Convert Bolt Torque to Clamping Force?
Clamping force is found with F = T / (K × D), where T is the applied torque, K is the friction coefficient (the nut factor) and D is the nominal bolt diameter. Only about 10 to 15 percent of the input torque becomes clamping force, with the rest lost to friction in the threads and under the bolt head.
This is why a dry bolt produces far less clamp than a lubricated one at the same torque. This calculator turns bolt torque into clamping force per bolt, total clamping force, bolt tensile stress and percent of proof load for Grade 5 and Grade 8 fasteners. Inputs include bolt size (M6 to M20 metric, 1/4″ to 3/4″ UNC), applied torque in ft-lbs, friction coefficient and the number of bolts in the joint.
Common nut factors and limits: dry steel K ≈ 0.20 | lightly oiled K ≈ 0.15 | lubricated or plated K ≈ 0.10 | Grade 5 proof load 85,000 psi | Grade 8 proof load 120,000 psi | target 60–75% of proof load for reusable joints. Example: an M8 bolt (D = 0.315″) at 25 ft-lbs with K = 0.15 yields about 6,350 lbf of clamping force.
How Bolt Clamping Force Works
When you tighten a bolt, you convert torque into tension. The bolt stretches like a spring. That stretch pulls the joint together and creates clamping force.
The Torque-Force Relationship
The formula F = T / (K x D) is simple but powerful. T is your applied torque in inch-pounds. K is the friction coefficient. D is the bolt diameter. Most of the torque you apply goes to fighting friction. Only about 10 to 15 percent becomes actual clamping force.
Friction Matters
Friction is the biggest variable. A dry bolt with K = 0.20 gives 25% less clamping force than an oiled bolt with K = 0.15 at the same torque. Always know your friction condition before setting torque values.
Pro tip: Use a consistent lubricant on bolt threads. This makes your clamping force more predictable. Inconsistent friction is the top cause of loose joints and broken bolts.
Proof Load
Proof load is the max force a bolt can take without permanent stretch. Grade 5 bolts handle 85,000 psi. Grade 8 handles 120,000 psi. For reusable joints, stay below 75% of proof load. For critical joints that get torqued once, you can go to 90%.
Multiple Bolts
Total clamping force equals the force per bolt times the number of bolts. Tighten bolts in a star pattern to spread the load evenly. Uneven tightening causes one bolt to carry more than its share.