Calculate bolt pretension, torque, diameter, or nut factor from any 3 inputs using the T = K × P × D formula in Nm, mm, and kN units.
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Bolt Pretension Formula
The bolt pretension calculator uses the torque-preload relationship between tightening torque, bolt diameter, nut factor, and bolt pretension.
Rearranged forms used to calculate each missing value:
- T = tightening torque, in N·m after unit conversion
- K = nut factor, dimensionless
- P = bolt pretension, in kN after unit conversion
- D = nominal bolt diameter, in mm after unit conversion
The calculator lets you enter any three values and solves for the fourth. If torque is missing, it multiplies nut factor, pretension, and diameter. If pretension is missing, it divides torque by nut factor and diameter. If diameter is missing, it divides torque by nut factor and pretension. If nut factor is missing, it divides torque by pretension and diameter.
The internal calculation uses N·m for torque, mm for diameter, and kN for pretension. This works because 1 kN·mm equals 1 N·m.
Typical Nut Factor Values
The nut factor, K, has a large effect on the calculated pretension. Use a value that matches the actual bolt, nut, washer, coating, and lubrication condition when available.
| Thread / bearing condition | Typical K range | Notes |
|---|---|---|
| Clean, dry steel | 0.20 to 0.30 | Higher friction, more torque lost to friction |
| Lightly oiled steel | 0.15 to 0.22 | Common estimate for general bolting |
| Lubricated threads and bearing face | 0.10 to 0.18 | Lower torque may create the same pretension |
| Plated or coated fasteners | 0.11 to 0.25 | Depends strongly on coating and lubricant |
Common Unit Conversions
| Quantity | Conversion |
|---|---|
| Torque | 1 lb-ft = 1.355817948 N·m |
| Diameter | 1 inch = 25.4 mm |
| Diameter | 1 cm = 10 mm |
| Pretension | 1 lbf = 0.0044482216 kN |
| Pretension | 1 kN = 224.8089439 lbf |
Example Problems
Example 1: Calculate bolt pretension
You have a torque of 120 N·m, a bolt diameter of 12 mm, and a nut factor of 0.20.
The bolt pretension is 50 kN.
Example 2: Calculate tightening torque
You want a pretension of 30 kN on a 10 mm bolt with a nut factor of 0.18.
The required tightening torque is 54 N·m.
FAQ
What is bolt pretension?
Bolt pretension is the tensile force created in a bolt when it is tightened. It clamps the joint members together. In this calculator, pretension is calculated from torque, nominal diameter, and nut factor.
Why does the nut factor matter so much?
The nut factor represents the combined effect of thread friction, bearing surface friction, lubrication, coating, and fastener condition. A lower K value means less torque is lost to friction, so the same torque creates more bolt pretension. A higher K value means more torque is lost to friction, so the same torque creates less pretension.
Is torque a precise way to set bolt pretension?
Torque is a common method, but it is not highly precise because friction can vary significantly. The calculated pretension is an estimate unless the nut factor is known from testing or a controlled bolting specification. For critical joints, use the tightening method and values specified by the applicable engineering standard or design procedure.
