Spring Rate Calculator

Enter the wire diameter, coil diameter, active coils and material to get the spring rate in N/mm and lb/in.

Spring rate calculator

Wire diameter (mm)

Mean coil diameter (mm)

Number of active coils

Material

Spring rate: —

Spring rate in pounds per inch: —

Spring index: —

—

The spring rate uses the standard formula k = G x d^4 / (8 x D^3 x N), with d the wire diameter, D the mean coil diameter and N the active coils.

Common questions about the free online spring rate calculator.

Spring rate k equals G times the wire diameter to the fourth power, divided by eight times the coil diameter cubed times the active coils.

G measures how stiff the wire material is in shear. Music wire is about 79.3 GPa, stainless steel about 69 GPa and phosphor bronze about 41.4 GPa.

Multiply the metric rate by 5.7101. A rate of 10 newtons per millimetre is about 57.1 pounds per inch.

Active coils are the free coils that deflect. Closed or ground end coils sitting flat on the seat do not count towards the total.

The spring index is the mean coil diameter divided by the wire diameter. Values between 4 and 12 are the easiest to manufacture and use.

Yes. The rate rises with the fourth power of wire diameter, so doubling the wire diameter makes the spring sixteen times stiffer.

Adding active coils makes the spring softer, because the rate is inversely proportional to the number of active coils in the spring.

No. The spring rate is calculated in your browser, so the dimensions and material you choose never leave your device.

Yes. It is free to use with no account, and you can check as many spring designs as you need.

Last updated: October 10, 2026