Calculate required bolt torque
The relation between applied torque and axial force - or load - in a bolt can be calculated as
T = K F d (1 - l/100) (1)
T = wrench torque (Nm, lbf ft)
K = constant that depends on the bolt material and size
d = nominal bolt diameter (m, ft)
F = axial bolt force (N, lbf)
l = fubrication factor (%)
Typical values for K with mild-steel bolts in range 1/4" to 1":
- normal dry: K = 0.2
- nonplated black finish: K = 0.3
- zinc-plated: K = 0.2
- slightly lubricated: K = 0.18
- cadmium-plated: K = 0.16
Note! - be aware that this is a rough calculation - in addition the accuracy of a torque wrench is normally no better than +-25%.
Bolt Torque Calculator
The calculator below can be used to calculate the torque required to achieve a given axial bolt force or load. The calculator is generic an can used for imperial and metric units as long as the use of units are consistent.
K - constant
d - diameter of bolt (m, ft)
F - axial bolt force or load (N, lbf)
Note that standard dry torques are normally calculated to produce a tensile stress - or axial force or clamp load - in the bolt that equals to 70% of minimum tensile strength or 75% of proof strength.
Example - Required torque for tightening a Imperial bolt
The required bolt clamping force for a joint is 20000 lbs. The torque required for a 3/4" dry steel bolt with 0% lubrication to achieve this tension can be calculated as
Tdry = (0.2) (20000 lb) (0.75 in) (1/12 ft/in) (1 - (0%) / (100%))
= 250 (lbf ft)
Example - Required torque for tightening a Metric bolt to proof load
The proof load for a M30 metric bolt grad 8.8 is 337000 N. The torque required to achieve this tension with a dry bolt with 0% lubrication can be calculated as
Tdry = (0.2) (337000 N) (30 mm) (10-3 m/mm)
= 2022 (Nm)
Lubricating the bolt with SAE 30 oil reduces the torque required to achieve the same tension with approximately 40%. The reduced torque can be calculated
TSAE30 = (2022 Nm) (1 - (40%) / (100%))
= 1213 Nm
Bolt Force vs. Torque
Eq. 1 can be rearranged to express bolt force as
F = T / (K d (1 - l / 100)) (1a)
Example - Dry vs. Lubricated Bolt
The proof load for a M30 metric bolt grad 8.8 is 337000 N. The torque required to achieve this force with a dry bolt is calculated to 2022 Nm.
By failure the bolt is lubricated and tightened with the same torque 2022 Nm. The force acting in the lubricated bolt can be calculated as
Flubricated = (2022 Nm) / (0.2 (0.03 m) (1 - (40%) / (100%)))
= 561667 N
This is way above what the bolt can handle and fatal failure is likely.
Bolt Force Calculator
This calculator can be used to calculate the force acting in a bolt.
T - torque (Nm, lbf ft)
K - constant
d - diameter bolt (m, ft)
torque reduction due to lubrication (%)