Bolt torque calculator (inch bolts)
An estimate of the tightening torque for an inch-thread bolt from its diameter, threads per inch, grade and lubrication, with the clamp load it gives.
Your numbers
The boltTightening See your resultResult
Torque 75.4 lb-ft102 N·m Clamp load 9,046 lbf Tensile stress area 0.1419 sq in Stop working this out by handFleetZap PM work orders arrive with their checklist, and steps can be required before the job closes, so a torque check can’t be skipped.
See FleetZap Maintenance · Try the demoThe formula
Torque (lb-ft) = K × Diameter × Clamp load ÷ 12 · Clamp load = Stress area × Proof strength × Preload
The stress area of an inch thread is π ÷ 4 × (D − 0.9743 ÷ n)². With the example, a 1/2-13 Grade 5 bolt, dry, tightened to 75% of proof load: about 9,050 lbf of clamp and 75 lb-ft. The nut factor swings the result a lot: oil, plating and thread condition all change it. Always use the equipment maker’s torque spec where one exists; this is an estimate for when it doesn’t.
Using the numberWhat to do with it
- Torque is only a stand-in for clamp load. Clean, undamaged threads and the right lubricant matter as much as the wrench.
- Lubricated bolts need less torque for the same clamp. Using a dry value on an oiled bolt can stretch or break it.
- Re-check torque on wheel nuts and critical joints after the first hours of use, as the maker’s manual says.
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Good to know
How do you calculate bolt torque?Torque = K × D × F, where K is the nut factor, D the diameter and F the clamp load. Divide by 12 for lb-ft when D is in inches. A 1/2-13 Grade 5 bolt, dry, at 75% of proof load is about 75 lb-ft.
What nut factor should I use?About 0.2 for dry, plain steel threads and about 0.15 for lubricated threads. Coatings and thread condition change it, which is why makers' specs come first.
Should I use this instead of the manual?No. Always use the equipment maker's torque spec where one exists. This calculator is an estimate for when no spec is given.
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