# Bolt Torque Calculator

## 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)*

*where *

*T = wrench torque (Nm, in lb)*

*K = constant that depends on the bolt material and size *

*d = nominal bolt diameter (m, in)*

*F = axial bolt force (N, lb)*

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* - zink-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, in)*

* F - axial bolt force or load (N, lb)*

* torque reduction due to lubrication (%)*

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 to achieve this tension can be calculated as

*T _{dry} = (0.2) (20000 lb) (0.75 in)*

* = 3000 (in lb)*

* = 3000 (in lb) / 12 (in/ft) *

* = 250 (ft lb) *

### Example - Required torque for tightening a Metric bolt to proof load

The proof load for a *M30* metric bolt grad 8.8 is *373000 N*. The torque required to achieve this tension with a dry bolt can be calculated as

*T _{dry} = (0.2) (373000 N) (30 mm) (10^{-3} m/mm) *

* = 2238 (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

*T _{SAE30} = (2238 Nm) (1 - (40%) / (100%))*

* = 1343 Nm*

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