You can either do it by saying the force is equal to the stress times the tensile stress area,
F = σ x A
where A = (pi/4)(D - 0.9382P)² (D= diameter in mm
and P is the thread pitch in mm
. Bear in mind that this can be defined differently in critical areas.
But a stress is just Young's modulus times the strain, and strain is the extension over the original length, so you can rearange the above to
F = E.δ.A/L
Where E is Young's modulus, δ is the extension, A is the tensile stress are and L is the effective bolt length. Remember to use the forumla above things need to be in SI
If you've got a few different sections, just split up the strains:
F = E.d/((L1
If you've got the torque, if I recall correctly you divide the torque by the effective area and something called the nut factor, which is dimensionless and varies with material and lubrication.
F = T/A.P
where P is the nut factor (between .08 and .5ish)