Spring Rate Converter

Convert spring rates between N/mm, N/m, kN/m, lb/in, lb/ft, kg/mm and kgf/mm — both directions, instantly. Plus a Hooke's law (F = k·x) solver, a series & parallel spring combination calculator, and a wheel-rate calculator from motion ratio, with automotive presets. Everything runs 100% in your browser; no data ever leaves your device.

Spring rate unit converter

Value

From

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Value in every unit

Unit	Value

Note: in suspension catalogs, "kg/mm" means kilogram-force per millimetre (kgf/mm) — an automotive convention, not mass per length. This converter treats the two identically (see FAQ).

Hooke's law solver — F = k · x

Enter any two of the three values; the solver computes the third. Units are converted automatically.

Solve for

Force F

Force unit

Spring rate k

Rate unit

Displacement x

Length unit

—

F = k · x → k = F / x → x = F / k. Linear-elastic range only: real springs deviate near coil bind and past yield.

Springs in series & parallel

Add up to 8 springs. Each spring can use its own unit — everything is converted internally.

Show results in

—

Series: 1/k = Σ(1/k_i) — softer than the softest spring

—

Parallel: k = Σk_i — stiffer than the stiffest spring

Series: 1/k_total = 1/k₁ + 1/k₂ + … Parallel: k_total = k₁ + k₂ + …

Wheel rate from motion ratio

The spring never sits exactly at the wheel — suspension leverage scales its effective rate by the square of the motion ratio.

Direction

Spring rate

Rate unit

Motion ratio (MR)

Typical motion ratios:

—

k_wheel = k_spring · MR² and k_spring = k_wheel / MR², where MR = spring travel ÷ wheel travel.

Automotive presets & reference table

Typical coilover spring rates used as starting references. Tap one to load it into the unit converter. Always verify against your corner weights and intended use.

Quick reference: 1 unit in the others

From	lb/in	N/mm	kgf/mm

Preset figures are approximate industry-typical starting points, not specifications for any particular vehicle. Aftermarket coilover catalogs list the spring itself; your wheel rate will be lower according to your motion ratio (see the Wheel Rate tab).

Formulas used

F = k · x — Hooke's law: force equals spring rate times displacement from free length.

1/k_total = 1/k₁ + 1/k₂ + … — springs in series (compliances add).

k_total = k₁ + k₂ + … — springs in parallel (rates add).

k_wheel = k_spring · MR² — wheel rate from spring rate and motion ratio.

Exact conversion constants: 1 lb = 4.4482216152605 N, 1 in = 0.0254 m, 1 ft = 0.3048 m, g = 9.80665 m/s². Hence 1 N/mm = 5.71015 lb/in and 1 kgf/mm = 55.9974 lb/in.

Scope boundary — what this tool does not do

Not a spring designer: it does not compute coil geometry (wire diameter, coil count, free length) from a target rate — it converts and combines rates you already have.
Not a damper tool: damping, rebound/compression valving, and ride-frequency tuning are separate problems this calculator does not cover.
Linear model only: progressive springs, dual-rate crossovers, and coil bind need rate curves, not the single-rate math used here.
Not vehicle-specific advice: presets are generic starting points; final rates depend on your corner weights, tyres, and use case.

Related tools on nrtechstudio.com: length converter (for spring displacement units) and weight-to-mass converter (for the kgf vs. kg mass/force distinction).

Frequently asked questions

What is spring rate?

Spring rate is the stiffness of a spring: the force needed to compress or extend it by one unit of distance. It is written as force per length, for example N/mm, lb/in, or kgf/mm, and it follows Hooke's law, F = k · x.

How do I convert N/mm to lb/in?

Multiply the N/mm value by 5.71015 to get lb/in. For example, 10 N/mm equals 57.1015 lb/in. The converter above does this conversion both ways, exactly, between all seven supported units.

How do I convert kg/mm to N/mm?

Multiply the kg/mm value by 9.80665 to get N/mm, because one kilogram-force equals 9.80665 newtons. For example, 10 kg/mm equals 98.0665 N/mm.

What is the difference between kg/mm and kgf/mm?

In spring and suspension catalogs there is none: "kg/mm" is shorthand for kilogram-force per millimetre (kgf/mm). Both equal 9.80665 N/mm. A true mass-per-length unit cannot describe stiffness, so this converter treats kg/mm and kgf/mm as identical.

How do springs in series combine?

For springs in series, add their compliances: 1/k_total = 1/k1 + 1/k2 + ... The combined rate is always softer than the softest spring in the chain. For example, 100 N/mm and 200 N/mm in series give 66.6667 N/mm.

How do springs in parallel combine?

For springs in parallel, simply add the rates: k_total = k1 + k2 + ... For example, 100 N/mm and 200 N/mm in parallel give 300 N/mm.

What is wheel rate and how is it calculated?

Wheel rate is the effective spring rate felt at the wheel after suspension leverage is taken into account. It equals the spring rate times the motion ratio squared: k_wheel = k_spring · MR². For example, a 100 N/mm spring with a 0.8 motion ratio gives a 64.0 N/mm wheel rate.

What is motion ratio?

Motion ratio is spring travel divided by wheel travel: MR = spring displacement ÷ wheel displacement. A motion ratio of 0.8 means the spring moves 8 mm for every 10 mm of wheel movement. Because the ratio is squared in the wheel-rate formula, small changes in motion ratio change the wheel rate a lot.

How do I use Hooke's law, F = k · x?

Hooke's law says force equals spring rate times displacement. Enter any two of the three values in the Hooke solver above and it calculates the third, converting units for you. Just keep the units consistent: for example, k in N/mm with x in mm gives F in newtons.

What spring rate do I need for my car?

It depends on corner weight, target ride frequency, and motion ratio. A common street-car starting point is a ride frequency around 1.5–2.0 Hz at the front and slightly higher at the rear. The presets tab lists typical coilover rates as a starting reference, but always verify against your actual corner weights and intended use.

Is a stiffer spring rate always better for handling?

No. Stiffer springs reduce body roll and sharpen response, but an overly stiff spring loses mechanical grip on bumpy surfaces, ruins ride quality, and can overwhelm the dampers. The right rate balances roll control, grip, and comfort for your tyres, weight, and roads.

Does temperature change spring rate?

Slightly. Steel coil springs lose roughly 1–3% of their rate per 100 °C of temperature rise because the shear modulus of steel drops with heat. The effect is negligible for normal road and track use.

Why do US and Japanese catalogs use different spring rate units?

US catalogs traditionally use lb/in (pounds per inch), while Japanese and European catalogs use kgf/mm, often written "kg/mm", or N/mm. They describe the same physical stiffness: 1 kgf/mm equals 55.9974 lb/in. This converter translates between them exactly.

Is this spring rate converter free to use?

Yes. It runs entirely in your browser: no account, no sign-up, and no data is sent anywhere. Unit conversion, the Hooke's law solver, series and parallel combinations, and wheel rate calculation are all free and unlimited.

Free tool by NRTechStudio

Spring Rate Converter

Spring rate unit converter

Value in every unit

Hooke's law solver — F = k · x

Springs in series & parallel

Wheel rate from motion ratio

Automotive presets & reference table

Quick reference: 1 unit in the others

Formulas used

Scope boundary — what this tool does not do

Frequently asked questions

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