Bolt Torque Calculator
Tightening torque, preload and ratio to yield from the bolt size, property class (4.6 to 12.9) and friction coefficient, by T = K·F·d and the simplified VDI 2230 method
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Tightening torque for a target preload from the bolt size, property class and friction coefficients, by T = K·F·d and by the simplified VDI 2230 relation, with the bolt stress as a share of the yield strength. For bolts that have no torque on the drawing.
How to use
- On "Calculator" choose the "Thread" and "Property class".
- Under "Start from" pick "Yield utilisation", "Given preload" or "Given torque" and enter the value.
- Set "Thread friction coefficient µG", "Head bearing friction coefficient µK" and "Torque coefficient K (simple formula)"; the "Typical values" buttons fill them in.
- Compare torque, preload, equivalent stress and "Ratio to yield" of both methods, or open "Torque table" for M3 to M36.
How it works
- Stress area As = π/4 × ((d2 + d3)/2)²; strengths are the minimum values of ISO 898-1:2013 Table 3. T = K × F × d; VDI 2230: T = F × (0.159·P + 0.577·µG·d2 + µK·Dkm/2); equivalent stress σ_red = √(σ² + 3τ²).
- The mean bearing diameter Dkm defaults to 1.3 × d; enter the real value for washers or flange heads.
- All results are estimates: real preload can differ by ±20 to 30 % or more with friction scatter and tool error. A torque on the drawing or in a standard takes precedence.
Questions and answers
- What is the tightening torque for an M10 class 8.8 bolt?
- With friction 0.12 and 70 % yield utilisation, VDI 2230 gives about 43 N·m and the simple formula with K = 0.2 about 52 N·m. The VDI assembly preload (ν = 90 %) corresponds to about 48 N·m.
- Why do the two methods differ?
- The simple formula lumps friction and geometry into one factor K; VDI 2230 calculates lead, thread friction and head friction separately. K = 0.2 corresponds roughly to a friction coefficient of 0.14 to 0.15.
- What yield utilisation should I use?
- VDI 2230 uses 90 % of yield for the equivalent stress; simple calculations often use 60 to 75 % in tension alone. The tool warns above 90 %.
- Which friction coefficient should I enter?
- About 0.12 for lightly oiled zinc-plated or black-oxide bolts, 0.14 to 0.20 dry, 0.08 to 0.10 with lubricants such as molybdenum disulfide.