Configuring a torque value.

[bibacux2001]

Can anyone clarify the equation for working out the torque value on a bolt. I know the diameter and hardness of material play a part and also the coating on the bolt (i.e. zinc passivated etc) but how do you eventually arrive at a figure for say an M16 bolt or srew. Also how does the addition of oil/grease effect the torque value and can you give an explanation of what the grease does.

[heathfield]

Have a look here.
Note that lubrication will lower the friction coefficient resulting in the same load pressure with less applied torque.
Think of grease as a jelly containing oil in suspension. Even under intense pressure, the oil forms a layer of slippery molecules between the two surfaces, reducing the friction.
The torque value equation will only provide an estimate at best. To arrive at the true value, for product specifiation, bolt manufacturers have to conduct practical tests on their products.