What is spring rate (spring constant) and how is it calculated for a helical compression spring?

Spring rate (k) is the force required to deflect a spring by unit length, measured in N/mm. For helical compression springs, k = Gd^4 / (8D^3N), where G is shear modulus, d is wire diameter, D is mean coil diameter, and N is number of active coils. Higher wire diameter and lower coil diameter increase stiffness. Spring rate is crucial for selecting springs that provide desired force at specified deflections.