What this calculator does
The Bolt Torque Calculator estimates the tightening torque you should apply to a bolt, and the clamp force (preload) that torque produces in the joint. It covers the two most common thread systems — metric (ISO) coarse threads with property classes from 4.8 to 12.9, and imperial (UNC) threads with SAE grades 2, 5, and 8 — and it takes lubrication into account, because the same torque produces very different bolt tension on a dry thread than on an oiled one.
The results follow the standard engineering relationship between torque, bolt size, and preload, using the tensile stress area of each thread size. The page runs entirely in your browser and is available in English, Arabic, and Spanish.
How to use the Bolt Torque Calculator
- Choose the thread system — metric (M sizes) or imperial (UNC sizes such as 1/2-13).
- Select the bolt size and its strength grade: the property class (for example 8.8 or 10.9) is normally stamped on the metric bolt head, and SAE grade lines are marked on imperial bolts.
- Choose the lubrication condition, which sets the nut factor K: dry threads use K = 0.20, lightly oiled threads use about 0.17, and well-lubricated threads use about 0.13.
- Choose the tightening level: 75% of the bolt's yield strength is the standard choice for joints that must be disassembled again; 90% is used only for permanent joints.
- Click "Calculate" to see the recommended torque, the preload (clamp) force, and the bolt's total yield load.
Formulas
With tensile stress area As, yield strength σy, utilization c (0.75 or 0.90), nut factor K, and nominal diameter d:
Metric bolts work in N and N·m with d in millimeters (converted to meters); imperial bolts work in lbf and ft·lb with d in inches. The tensile stress areas used are the standard ISO and UNC values for coarse threads, so fine-pitch bolts will give slightly different results.
Examples
- An M12, class 8.8 bolt, dry (K = 0.20), tightened to 75% of yield: preload F = 0.75 × 640 MPa × 84.3 mm² ≈ 40,500 N, and torque T = 0.20 × 40,500 × 0.012 m ≈ 97 N·m (about 72 ft·lb).
- A 1/2-13 UNC, Grade 5 bolt, lightly oiled (K = 0.17), at 75% of yield: F ≈ 9,790 lbf, giving T ≈ 70 ft·lb (about 94 N·m).
- An M10, class 10.9 bolt, well lubricated (K = 0.13), at 75% of yield: F ≈ 40,900 N and T ≈ 53 N·m — the same bolt dry would need about 82 N·m for the same preload.
Frequently asked questions
Why does lubrication change the torque?
Most of the wrench torque is lost to friction under the nut and in the threads. Lubrication reduces that friction, so a lower torque produces the same bolt tension — and if you apply the dry-thread torque to a lubricated bolt you can easily over-stretch and break it. That is why the nut factor K is a required input.
What is the nut factor K?
K is a single lumped friction coefficient that combines thread friction, bearing friction, and thread geometry. There is no exact universal value: 0.20 is typical for dry plain bolts, 0.15–0.18 for lightly oiled or plated bolts, and 0.10–0.14 for well-lubricated ones. Measuring the actual bolt tension with a torque-tension tester is the only precise method.
Should I tighten to 75% or 90% of yield?
Use 75% for joints you may need to take apart, because it keeps a safety margin against over-stretching. 90% is reserved for permanent connections where maximum clamp load is wanted and the bolt will not be reused.
Why does my torque wrench table show a different value?
Published torque tables assume specific friction conditions and utilization factors, and they often include tolerances. Small differences in K (±0.02) change the result by 10% or more. Treat this calculator's result as a well-founded estimate and always give priority to the equipment manufacturer's specifications.
Can I use it for critical or safety-related joints?
For safety-critical connections — brakes, steering, lifting equipment, pressure vessels — always use the manufacturer's torque specification and procedure. This calculator is a general engineering estimate, not a substitute for the documented spec.
Is my data saved or sent anywhere?
No. Every calculation runs in your browser, and nothing you type is stored or sent to a server.
Related calculators
Other free calculators commonly used with this tool: Gear Ratio Calculator, Belt & Pulley Calculator, and Cutting Speed & Feed Calculator.
Want a deeper walkthrough? Read our bolt torque guide.