This free bolt torque calculator from HAGUE FASTENERS LIMITED works out the tightening torque and preload for metric coarse and fine, UNC and 8UN, UNF, BSW and BSF threads. Choose a property class or bolting grade such as ASTM A193 B7, B16, L7 or B8M, stainless steel, duplex or Inconel 718, set the lubrication, and see the torque in N m and lbf ft with the clamp load it gives.
Bolt torque and preload calculator
Torque from the ISO 16047 / VDI 2230 torque equation: T = F × (0.16P + 0.58μthd2 + μbDkm/2). “Percentage of yield” uses the VDI 2230 combined stress method, which allows for the twisting stress in the bolt while it is tightened. Friction values are typical only: use your lubricant maker’s data where you have it. Yield values are specification minimums and vary with diameter.
Guidance only. Results are approximate and for initial guidance. They are not a substitute for the governing standard, specification or drawing, or for testing. HAGUE FASTENERS LIMITED accepts no liability for decisions made using these tools. Always check critical values against the relevant standard. © 2026 HAGUE FASTENERS LIMITED.
How to use the bolt torque calculator
- Choose the thread type and size.
- Choose the grade or material. Yield strength fills in from the specification for that size, or choose Other and enter your own.
- Choose the lubrication, or type your own thread and under-head friction values.
- Work to a percentage of yield, or enter the preload you need.
- Check the nut across flats and clearance hole sizes, which affect the under-head friction.
How the calculation works
Torque comes from the torque equation used in ISO 16047 and VDI 2230: T = F × (0.16P + 0.58μthd2 + μbDkm/2), where F is preload, P is pitch, d2 is pitch diameter and Dkm is the mean bearing diameter under the nut. When you work to a percentage of yield, the calculator uses the VDI 2230 combined stress method, which allows for the twisting stress in the bolt while it is being tightened. Simpler calculators ignore this and can overstate the safe preload. For Whitworth threads it allows for the 55 degree thread angle.
Frequently asked questions
Why does lubrication change the torque so much?
Typically around 85 to 90% of the tightening torque is used overcoming friction in the threads and under the nut, and only around 10 to 15% stretches the bolt. A small change in friction therefore makes a big change to the preload you get for a given torque. The calculator shows where the torque goes for your inputs.
How accurate is torque tightening?
Even with a calibrated torque wrench, the preload achieved can vary by 25% or more because friction varies from bolt to bolt. For critical joints, such as pressure flanges, follow your joint assembly procedure, for example ASME PCC-1, and consider hydraulic tensioning.
What percentage of yield should I tighten to?
90% of yield (combined stress) is a common maximum for general structural joints. Flanged and gasketed joints often specify a lower target bolt stress. Always follow the design or procedure for your joint.
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