Calculate the spring rate (spring constant) of a helical compression coil spring by entering the wire diameter, mean coil diameter, number of active coils, and shear modulus. Get results in lbs/in, N/mm, and kg/mm simultaneously — useful for automotive suspension, engineering, and DIY spring identification.
Spring Rate
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Spring Rate (N/mm)
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Spring Rate (kg/mm)
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Mean Coil Diameter
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Spring Index (C = D/d)
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Spring rate (also called the spring constant) measures how much force is required to compress or extend a spring by one unit of distance. It is expressed in lbs/in, N/mm, or kg/mm. A higher spring rate means a stiffer spring. For automotive suspensions, spring rate directly affects ride comfort, handling, and body roll.
The calculator uses the standard helical compression spring formula: k = (G × d⁴) / (8 × D³ × N), where G is the shear modulus of the material, d is the wire diameter, D is the mean coil diameter (inside diameter + wire diameter), and N is the number of active coils. This is the universally accepted formula in mechanical engineering.
The inside diameter is the opening through the center of the spring. The mean coil diameter is the average diameter measured at the centerline of the wire — it equals the inside diameter plus one wire diameter. The spring rate formula requires the mean coil diameter, so the calculator computes it automatically from your inside diameter input.
Active coils are the coils that actually flex under load. For compression springs with closed (ground) ends, subtract the two inactive end coils from the total coil count. For example, a spring with 10 total coils and 2 closed ends has 8 active coils. Open-ended springs count all coils as active.
Most automotive suspension springs are made from high-carbon steel or chrome-silicon alloy steel. Use 11,500,000 psi (79.3 GPa) for standard carbon steel or 6,500,000 psi (44.8 GPa) for chrome-silicon. Stainless steel springs use approximately 10,500,000 psi. When in doubt, standard steel (11.5M psi) is the most common choice.
Real-world spring rate can vary from theoretical calculations due to material inconsistencies, manufacturing tolerances, end coil geometry, and the effects of spring set (permanent deformation over time). The formula assumes ideal conditions, so results should be used as close approximations and verified with physical testing when precision is critical.
Spring index (C) is the ratio of the mean coil diameter to the wire diameter (C = D/d). It indicates how tightly wound a spring is. A spring index below 4 is difficult to manufacture and prone to high stress concentrations. An index between 4 and 12 is generally considered ideal, while values above 12 may indicate a fragile or difficult-to-handle spring.
To convert: 1 lbs/in = 0.17513 N/mm, and 1 lbs/in = 0.017857 kg/mm. Alternatively, 1 N/mm = 5.7102 lbs/in and 1 kg/mm = 56.0 lbs/in. This calculator automatically displays all three units so you can use whichever your spring brand or datasheet requires.