Spring Rate Converter

Understanding Spring Rate

Spring rate (also called spring constant or stiffness) measures how much force is needed to compress or extend a spring by a unit of distance. It is a fundamental property used in automotive suspension design, industrial machinery, and mechanical engineering. A higher spring rate means a stiffer spring that requires more force to deflect.

The Three Common Units:

• lb/in (pounds per inch): Used primarily in the United States for automotive and general-purpose springs. A 500 lb/in spring requires 500 pounds of force to compress it one inch.
• N/mm (Newtons per millimeter): The SI (metric) standard used internationally in engineering specifications. Also written as kN/m (kilonewtons per meter), where 1 N/mm = 1 kN/m.
• kg/mm (kilograms per millimeter): Common in Japanese and European automotive aftermarket parts. Technically measures force in kilogram-force (kgf), not mass.

Conversion Formulas:

1 lb/in = 0.17513 N/mm 1 lb/in = 0.017858 kg/mm 1 N/mm = 5.7101 lb/in 1 N/mm = 0.10197 kg/mm 1 kg/mm = 9.8067 N/mm 1 kg/mm = 55.997 lb/in

These conversions combine both force unit conversion and length unit conversion. For example, converting lb/in to N/mm requires converting pounds to Newtons (multiply by 4.44822) and inches to millimeters (divide by 25.4).

Automotive Suspension Examples:

Hooke’s Law:

Spring rate is defined by Hooke’s Law: F = k × x, where F is force, k is the spring rate, and x is the deflection distance. This law holds true as long as the spring is not compressed beyond its elastic limit (fully compressed or “coil bound”).

Rearranged for the question people actually ask, it becomes x = F ÷ k. Put a load into the field above and the calculator does exactly that: a 700 lb corner on a 500 lb/in spring compresses it 1.4 inches. Double the rate and the travel halves; the two are strictly inverse, which is the whole reason a stiffer spring feels like it has less suspension.

Wheel rate is not spring rate

Worth knowing before you order anything on the strength of a number. What the car feels at the tyre is the wheel rate, and that is the spring rate multiplied by the square of the motion ratio: wheel rate = spring rate × MR².

On a strut car where the spring sits almost directly over the wheel the motion ratio is close to 1 and the two are nearly the same. On a double wishbone with an inboard spring the ratio might be 0.6, and 0.6² is 0.36, so a 500 lb/in spring behaves like 180 lb/in at the tyre. That squared term catches people out constantly, because halving the motion ratio quarters the effective rate rather than halving it.

Practical Notes:

• Lowering spring rates improves ride comfort but reduces handling precision
• Raising spring rates improves cornering response but makes the ride harsher
• Progressive-rate springs start soft and get stiffer as they compress, offering a compromise
• When comparing springs from different regions, always convert to the same unit before comparing