Accepted formulae relating torque and tension, based on many tests are:-
For Imperial Sizes
M = P x D M = torque lbf·ft P = bolt tension lbf D = bolt diameter (ins)
For Metric Sizes
M = P x D M = torque N·m P = bolt tension Newtons D = bolt diameter (mm)
These formulae may be used for bolts outside the range of the tables
Formulae for Calculating the Effect of Torque Wrench Extensions
M1 = M2 x L1/L2 Where L1 is the normal length and L2 is the extended length, M1 is the set torque and M2 the actual torque applied to the nut.
The required torque on the fastener is 130 N·m (M2) but what do you set on the torque wrench scale?
L1 = 500 L2 = 650
(units of length not important, this is ratio)
M1 = 130 x 500/650
M1 = 100
The Torque Wrench Extension Calculator can do all the calculations for you.