Coil spring rate

The formula for calculating the stiffness or rate of a coil spring.

Spring rate (K)=wire dia (d) to 4th power times Spring modulus 12.000.000 all divided by 8 times the number of active wraps(N)times Diameter of the coil(D) cubed

. t

K = Spring rate in pounds per inch

W = Diameter of the spring wire in inches

G = 12,000,000 for steel springs (a constant)

N = Number of active coils (number of coils that are free to move + 1/2 coil)

D = Diameter of the coils measured to the center of the wire, in inches

Note the relationship between the number of active coils (N) and the spring rate (K).

If you cut off half the active coils the spring rate is doubled.

source