🔩 Bolt Torque Calculator
Pick the bolt, its property class and lubrication: get the tightening torque and preload calculated with VDI 2230, plus a reference table and a torque unit converter.
⚠️ Indicative values, with no safety factor. Always use the torque given by the manufacturer (engine, wheels, structure) and a calibrated torque wrench.
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About Bolt Tightening Torque
Tightening a bolt stretches it like a spring so that it clamps the parts together: that is the preload. Too low and the joint works loose; too high and the bolt yields or breaks. Only part of the wrench torque becomes preload, as most of it is lost to friction in the threads and under the head. This tool works out the torque that brings the bolt to a chosen percentage of its yield strength, using the VDI 2230 method.
How to Use the Tool
- Pick the thread (M3 to M36, coarse or fine pitch) and the class stamped on the head (8.8, 10.9…).
- Choose the lubrication: it is the parameter that changes the result the most.
- Keep 90% utilisation to match the usual tables, or lower it for a more cautious tightening.
- Read the torque in N·m and other units; the “Convert a torque” tab converts any value.
Formulas Used (VDI 2230)
- Preload: F = As × ν × Rp0.2 ÷ √(1 + 3 × [3/2 × d2/d0 × (P ÷ (π × d2) + 1.155 × µG)]²), with As the stress area, ν the utilisation and Rp0.2 the yield strength (ISO 898-1, ISO 3506-1).
- Torque: M = F × (0.16 × P + 0.58 × d2 × µG + Dkm/2 × µK), with P the pitch, d2 the pitch diameter and Dkm the mean bearing diameter under the head.
- Example: M10 class 8.8, µ = 0.12, 90%: about 29.6 kN of preload and 48 N·m, as in the tables published from VDI 2230.
- K factor: the tool also shows the K of the simplified formula M = K × F × d (about 0.16 at µ = 0.12 and 0.21 at µ = 0.16) for comparison.
Tips
- Never mix dry and lubricated: a dry torque applied to a greased bolt can break it.
- Tighten gradually and in a cross pattern for multi-bolt joints (flanges, cylinder heads, wheels).
- Wheel nuts, cylinder heads, brakes: only use the vehicle manufacturer’s value.
- Stainless: use an anti-seize paste and tighten slowly.
Common Mistakes and Limits
- Uncertain friction: µ easily ranges from 0.08 to 0.20, which can double the torque.
- Other heads or washers: a socket head, a large washer or a tight hole change the bearing diameter.
- Wrench scatter: a torque wrench is often ±4% accurate; the actual preload varies much more than the torque.
- No joint analysis: the tool checks neither the clamped parts nor bolt fatigue.
Privacy & Security
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- No sign-up: free and instant.
- Works offline once the page has loaded.