Spring Rate Calculator
Calculate the stiffness of a helical compression spring from wire diameter, coil diameter, and active coils.
How it works
- 1Enter wire diameter, mean coil diameter, and the number of active coils, then select the spring material.
- 2The calculator applies k = G·d⁴ / (8·D³·n) in SI units, where G is the material shear modulus.
- 3Results are shown in N/m, N/mm, and lbf/in so you can use the value directly in any unit system.
Use cases
- Designing suspension springs for vehicles or motorcycles to achieve a target ride frequency.
- Sizing return springs in pneumatic cylinders or valve assemblies.
- Verifying purchased spring specifications against a required stiffness target.
Frequently asked questions
What does spring rate mean?
Spring rate (k) is the force required to compress or extend a spring by one unit of length. A rate of 10 N/mm means each millimetre of deflection adds 10 N of force, per Hooke’s law F = k·x.
Why does wire diameter affect stiffness so strongly?
Wire diameter appears raised to the 4th power in k = G·d⁴/(8·D³·n). Doubling the wire diameter therefore increases stiffness by a factor of 2⁴ = 16, making it the most influential dimension.
What is the rate of a steel spring with d = 2 mm, D = 20 mm, n = 8?
With G for steel = 79.3 GPa, k = 79.3×10⁹ × (0.002)⁴ / (8 × (0.020)³ × 8) ≈ 2478 N/m ≈ 2.48 N/mm ≈ 14.1 lbf/in.
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