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Calcimator

Bolt Torque Calculator

Calculate required bolt tightening torque to achieve a desired clamping force using the T = K×D×F short-form torque equation.

Inputs

psi
lb

Results

Required Torque

375 in·lb

Required Torque

31.3 ft·lb

Preload Force5,000 lb
Bolt Stress35,236 psi
% of Proof Load41.5%
Tensile Stress Area0.14 in²
How to Use This Calculator
  1. Select the Bolt Diameter from the dropdown — standard UNC sizes from 1/4" to 1-1/2".
  2. Enter the Proof Load Stress in psi: SAE Grade 5 ≈ 85,000 psi, Grade 8 ≈ 120,000 psi.
  3. Set the Nut Factor (K): use 0.20 for dry steel, 0.15 for lubricated threads, 0.12 for anti-seize.
  4. Enter your Desired Clamping Force in lb — typically 75% of the bolt proof load for static joints.
  5. Read the Required Torque in in·lb and ft·lb, and apply it with a calibrated torque wrench.
  6. Check the % of Proof Load to confirm you are not over-stressing the bolt (keep below 85–90%).

How the result changes with Nut Factor (K)

Nut Factor (K)Required TorqueRequired Torque
0.08200 in·lb16.7 ft·lb
0.16387.5 in·lb32.3 ft·lb
0.25612.5 in·lb51 ft·lb
0.32800 in·lb66.7 ft·lb

What each input means

Bolt Diameter
Nominal bolt diameter. Select from standard UNC bolt sizes.
Proof Load Stress
Proof load stress of the bolt. SAE Grade 5 ≈ 85,000; Grade 8 ≈ 120,000; A325 ≈ 85,000 psi.
Nut Factor (K)
Nut factor (torque coefficient). Dry steel ≈ 0.20; lubricated ≈ 0.15; anti-seize ≈ 0.12; cadmium plated ≈ 0.10.
Desired Clamping Force
Required clamp load to maintain joint integrity. Typically 75% of bolt proof load for static joints.

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    Bolt Diameter = 0.5, Proof Load Stress = 85000, Nut Factor (K) = 0.15, Desired Clamping Force = 5000 = 4 input(s) provided
  2. Calculate Required Torque
    Required Torque
    375 = 375
  3. Calculate Required Torque
    Required Torque = round((requiredTorque / 12) * 10) / 10
    31.3 = 31.3
  4. Calculate Preload Force
    Preload Force
    5000 = 5000
  5. Calculate Bolt Stress
    Bolt Stress
    35236 = 35236

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