Suspension Spring Rate Calculator
The rate of a helical coil spring tells you how stiff it is: how much force it takes to compress it by a unit of length. For a round-wire compression spring the rate follows a well-established mechanics formula driven by the wire diameter, the mean coil diameter, the number of active coils, and the shear modulus of the material. Because the wire diameter enters to the fourth power, small changes in wire size produce large changes in rate. This calculator uses the standard formula and lets you set the shear modulus and coil counts as user-editable inputs so it matches your actual spring and material.
Spring rate formula
k = (G * d^4) / (8 * D^3 * n)
d = wire diameter, D = mean coil diameter
n = active coils, G = shear modulus
Spring index C = D / d
1 lb/in = 0.175127 N/mm
The rate is the force per unit deflection. Force at any deflection is rate times deflection, so the force at one inch equals the rate in lb/in. Spring index C should normally fall between about 4 and 12 for manufacturable springs.
Spring design notes
- Wire diameter dominates: doubling d raises the rate sixteen-fold.
- Use the mean coil diameter (center to center of wire), not the outside diameter.
- Count only active coils; ground or closed end coils do not deflect.
- A spring index between about 4 and 12 is practical to wind and durable.
- Confirm the shear modulus for your exact alloy with the material supplier.
Spring rate: frequently asked questions
What is spring rate?
Spring rate is the force needed to compress a spring by one unit of length, for example pounds per inch or newtons per millimeter. A higher rate means a stiffer spring. For a helical compression coil spring it is set by the wire diameter, the mean coil diameter, the number of active coils, and the material's shear modulus.
What shear modulus should I use?
For common spring steels the shear modulus G is about 11.5 million psi (roughly 79.3 GPa). The value depends on the alloy, so it is a user-editable input. Use the figure your material supplier provides for an accurate result.
What counts as an active coil?
Active coils are the coils free to deflect under load. The end coils that are ground or closed do not deflect and are excluded. A common rule is active coils equal total coils minus about two for squared and ground ends, but always use the count for your specific spring.
What is the spring rate formula?
For a round-wire helical compression spring, rate k = (G times d to the fourth power) divided by (8 times D cubed times n), where d is wire diameter, D is mean coil diameter, n is the number of active coils, and G is the shear modulus. The rate is extremely sensitive to wire diameter because it enters to the fourth power.
Does mean coil diameter mean the outside diameter?
No. Mean coil diameter is measured center to center of the wire, which equals the outside diameter minus one wire diameter. Using the outside diameter by mistake makes the spring look softer than it is.
Official sources
- SAE International: spring design standards.
- U.S. National Institute of Standards and Technology: materials properties references.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.