Enter the torque applied to the bolt, the diameter of the bolt, and the nut factor (K) to determine the bolt pretension. This calculator helps in ensuring that bolts are properly tightened in mechanical assemblies.

Bolt Pretension Calculator

Enter any 3 values to calculate the missing variable

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Bolt Pretension Formula

The following formula is used to estimate the bolt pretension from tightening torque using the nut factor method.

P = T / (K * D)

Variables:

  • P is the bolt pretension (kN)
  • T is the torque applied to the bolt (N·m)
  • D is the nominal diameter of the bolt (mm)
  • K is the nut factor (also called torque coefficient), a dimensionless empirical value that depends strongly on friction/lubrication and joint conditions (often around 0.18–0.25, with ~0.20 commonly used for a rough estimate)

To calculate the bolt pretension, divide the torque applied to the bolt by the product of the nut factor (K) and the nominal diameter of the bolt (D). With T in N·m, D in mm, and P in kN, the units are consistent because 1 kN·mm = 1 N·m.

What is Bolt Pretension?

Bolt pretension is the tension applied to a bolt when it is tightened. This tension creates a clamping force that holds the bolted joint together. Proper bolt pretension is crucial for the integrity of the joint, as it helps resist joint separation and reduces the likelihood of loosening under operational loads. It is an essential parameter in the design and maintenance of mechanical assemblies.

How to Calculate Bolt Pretension?

The following steps outline how to calculate the Bolt Pretension.

  1. First, determine the torque applied to the bolt (T) in Newton-meters (N·m).
  2. Next, determine the nominal diameter of the bolt (D) in millimeters (mm).
  3. Next, estimate or determine the nut factor (K) (also called the torque coefficient). For many common steel bolt joints, a rough estimate is K ≈ 0.20, but it varies significantly with lubrication and surface condition.
  4. Finally, calculate the Bolt Pretension (P) in kilonewtons (kN) using the formula P = T / (K * D).
  5. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

torque applied to the bolt (T) = 50 N·m

nominal diameter of the bolt (D) = 10 mm

nut factor (K) = 0.20

Solution: P = T / (K · D) = 50 / (0.20 · 10) = 25 kN