Spring rate is determined by:
-Number of active coils
-Mean diameter
-Wire Thickness
-Modulus of rigidity (same for 99% of the springs)
You can calculate it here:
http://www.engineersedge.com/spring_comp_calc_k.htm
http://www.engineersedge.com/calcula...ring_k_pop.htm
How to determine the amount of active coils:
http://springipedia.com/compression-counting-coils.asp