Tightening a bolt isn't really about torque — it's about the invisible tension you create inside it. That tension, the preload or clamp force, is what actually holds a joint together, resists vibration and prevents fatigue failure. Torque is just the means to get there, linked to preload by one deceptively simple equation: T = K · D · F. This free Bolt Torque & Preload Calculator turns a target clamp load into the tightening torque you need — for any metric bolt grade, diameter and lubrication — and checks the bolt stress against its proof and yield strength.
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The Bolt Torque & Preload Calculator
Pick a metric bolt size and grade, choose a target preload (% of proof load) and a lubrication condition, and get the tightening torque, clamp force, bolt stress and proof-load utilisation. SI units.
Bolt Torque & Preload Calculator
The T = K D F Equation
The workhorse equation of bolted-joint engineering relates tightening torque to the preload it produces:
where T is the tightening torque (N·m), K the nut factor (dimensionless), D the nominal bolt diameter (m), and F the target preload / clamp force (N). It's beautifully simple — but its accuracy lives or dies by K, because any error in the nut factor passes directly into the preload.
Preload & Clamp Force
Preload (F) is the tension locked into the bolt when you tighten it — and it's the whole point of the joint. Preload:
- Clamps the joint members together
- Resists self-loosening under vibration
- Prevents fatigue by keeping the bolt in steady tension
- Seals gaskets and flanges against leaks
The target preload comes from the bolt's proof load:
where At is the tensile stress area and Sp the proof strength. The standard target is 75% of proof load for reusable joints. This same clamp-force thinking governs bolted flange joints in piping — where you also need the pipe wall thickness & hoop stress to size the whole assembly.
The Nut Factor (K)
The nut factor K lumps all the joint friction into one number. It's set almost entirely by lubrication and surface finish — not by how hard you pull:
| Condition | Nut factor K |
|---|---|
| Dry, plain steel | 0.20 |
| Lightly oiled | 0.15 |
| Moly grease (MoS₂) | 0.10 |
| Waxed / PTFE | 0.12 |
| Hot-dip galvanized | 0.18 |
| Stainless on stainless | 0.30 |
Why Lubrication Changes Everything
Here's the surprise: in a typical joint, only about 10% of your torque actually stretches the bolt. The rest is eaten by friction:
- ~50% lost to friction under the bolt head / nut face
- ~40% lost to friction in the threads
- ~10% actually produces clamp load
Bolt Grades & Tensile Stress Area
The property class sets the strengths; the tensile stress area At (a little larger than the minor-diameter area) sets how much load the bolt carries:
| Class | Proof (MPa) | Yield (MPa) | Tensile (MPa) |
|---|---|---|---|
| 4.8 | 310 | 340 | 420 |
| 5.8 | 380 | 420 | 520 |
| 8.8 | 580 | 640 | 800 |
| 10.9 | 830 | 940 | 1040 |
| 12.9 | 970 | 1100 | 1220 |
Typical ISO coarse-thread stress areas: M8 = 36.6, M10 = 58.0, M12 = 84.3, M16 = 157, M20 = 245 mm². A higher grade allows a higher preload before yielding — so it needs more torque for the same bolt size.
Worked Example
An M12 class 8.8 bolt, dry (K = 0.20), target 75% of proof load:
- At = 84.3 mm²; proof strength Sp = 580 MPa
- Preload F = 0.75 × 84.3 × 580 ≈ 36,700 N = 36.7 kN
- Torque T = 0.20 × 0.012 × 36700 ≈ 88 N·m (65 lbf·ft)
- Bolt stress = 36700 / 84.3 ≈ 435 MPa — comfortably below the 640 MPa yield.
Switch to moly grease (K = 0.10) and the same 36.7 kN preload needs only ~44 N·m — half the torque.
Common Mistakes
- Torquing lubricated bolts to dry values. Over-tensions and can snap them — match K to the lube.
- Treating torque as preload. Preload holds the joint; torque only generates it (with big scatter).
- Ignoring the ±25% torque scatter. Design critical joints for the min/max preload range.
- Using the wrong stress area. Use the tensile stress area At, not the shank area.
- Exceeding proof load. Above ~90% proof, only with stretch/angle control on permanent joints.
- Lubricating threads but not the bearing face. Mixed friction makes K — and preload — unpredictable.
- Reusing yielded bolts. Once past yield, discard — the preload capacity is gone.
Frequently Asked Questions
What is the bolt torque formula?
T = K · D · F — torque equals the nut factor times nominal diameter times preload. K (~0.10–0.30) captures friction; any error in it passes straight into the preload.
What is bolt preload?
The tension (clamp force) created when a bolt is tightened. Preload — not torque — holds the joint, resists loosening and prevents fatigue. Target is usually ~75% of proof load.
What is the nut factor K?
An empirical friction coefficient in T = K·D·F: ~0.20 dry, 0.15 oiled, 0.10 moly, 0.18 galvanized, 0.30 stainless. Lubrication roughly halves the torque for a given preload.
Why does lubrication change the torque so much?
Only ~10% of torque stretches the bolt; ~90% fights friction (50% head, 40% threads). Cutting friction sharply lowers the torque for the same preload — so lubricated bolts need less.
What percentage of proof load should I use for preload?
About 75% of proof load for reusable joints; up to 85–90% for permanent joints with angle/stretch control. Lower for non-critical joints.
How accurate is torque control of preload?
Only about ±25%, because preload depends on friction (K). Angle control, bolt-stretch measurement or tension-indicating washers are more precise.
Conclusion
A bolted joint holds because of preload, and torque is simply how you create it — tied together by T = K · D · F. Get the nut factor right for your lubrication, target about 75% of proof load, use the correct tensile stress area, and always allow for the ±25% scatter of torque control. Use the calculator above to convert any target clamp load into a tightening torque, and to check the bolt stays safely below yield.
For more engineering, fluid mechanics and CFD tutorials plus free calculators, explore Free CFD Tutorial. If this tool helped you, please share it with your fellow engineers and students.
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