What is the Bolt Torque Calculator?
Calculate bolt tightening torque and clamp preload with this free bolt torque calculator. Use T = K × D × F for torque from preload, solve preload from applied torque, or get recommended torque from bolt proof strength and tensile stress area. Find nut factor K from calibration tests. M6–M24 and 1/4"–3/4" presets, metric grades 8.8/10.9/12.9 and Grade 5/8. Results in N·m and lbf·ft. Runs instantly in your browser.
How to use the Bolt Torque Calculator
- Choose Torque from Preload, Preload from Torque, Recommended Torque, or Nut Factor mode.
- Select metric or imperial units.
- Enter nominal bolt diameter, nut factor K, and preload or torque as required.
- For recommended torque, set stress area, proof strength, and proof load percentage.
- Review torque, clamp force, and copy calculation steps.
Common use cases
- Setting torque wrench value for a structural bolt
- Estimating clamp force from a known tightening torque
- Sizing torque for Grade 8 or 10.9 fasteners
- Determining nut factor K from a calibration test
- Mechanical engineering homework on fastener preload
Frequently asked questions
- What is the bolt torque formula?
- T = K × D × F, where T is torque, K is the nut factor (friction coefficient), D is nominal bolt diameter, and F is axial preload (clamp force). Units must be consistent — e.g. N·m = K × m × N.
- What nut factor K should I use?
- Typical values: dry unplated steel K ≈ 0.20, zinc plated K ≈ 0.22, lubricated K ≈ 0.15, molybdenum disulfide K ≈ 0.12. Actual K varies with finish and lubrication — calibrate when possible.
- How is recommended torque calculated?
- Preload F = (% proof) × proof strength × tensile stress area A_s. Then T = K × D × F. A common target is 75% of proof load for structural bolts.
- What is tensile stress area?
- A_s is the effective cross-section at the thread root, smaller than the shank area. Torque tables use A_s — this tool includes presets for common metric and UNC bolt sizes.
- Does this account for thread pitch and washer friction?
- The K-factor method lumps all friction into one empirical constant. For critical joints, use manufacturer tables, torque-angle tightening, or strain-gauge calibration.
