Spring Rate Converter (N/mm, kgf/mm, lbf/in)

Understanding the spring rate converter — concept and principle

This converter changes a spring rate (stiffness) between N/mm, N/m, kgf/mm, kgf/cm, lbf/in and lbf/ft. Car and motorcycle enthusiasts comparing coil-over springs, mechanical engineers and students use it, since manufacturers quote rates in different unit systems.

Force per distance, re-unitised — with the default inputs: 447.979 lbf/in.

How it is calculated

Spring rate k = force ÷ deflection
All values go through N/m: 1 N/mm = 1,000 N/m; 1 kgf/mm = 9,806.65 N/m; 1 kgf/cm = 980.665 N/m
1 lbf/in = 175.12683 N/m; 1 lbf/ft = 14.5939 N/m
These follow from 1 kgf = 9.80665 N (exact), 1 lbf = 4.4482216152605 N and 1 in = 25.4 mm

The principle

A spring rate says how much force it takes to compress or extend the spring by one unit of length. Converting it means converting the force unit and the length unit together. The kilogram-force is the weight of 1 kg under standard gravity, 9.80665 m/s², so multiplying a kgf/mm value by 9.80665 gives N/mm. The calculation assumes a linear spring; progressive springs change rate along their travel.

Worked example

Defaults: 8 kgf/mm to lbf/in.

In N/m = 8 × 9,806.65 = 78,453.2; in N/mm = 78.4532.
lbf/in = 78,453.2 ÷ 175.12683 = 447.979 lbf/in.
kgf/cm = 8 × 10 = 80; lbf/ft = 5,375.75; kgf/mm 8.

Each point is the calculator re-run with only “Spring rate” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Spring rate	Result
2	111.995 lbf/in
4	223.99 lbf/in
6	335.984 lbf/in
8	447.979 lbf/in
10	559.974 lbf/in
12	671.969 lbf/in
16	895.959 lbf/in

How to read the result

A rate of 8 kgf/mm means each millimetre of compression needs about 78 N more force. Comparing springs from different makers only makes sense in the same unit. Remember that the rate at the wheel differs from the spring rate because of the suspension's motion ratio.

Common mistakes

Treating kgf as kg of mass.
Converting the force unit but forgetting the length unit.
Comparing a progressive spring's initial rate with a linear spring's rate.
Confusing spring rate with preload.

Keep in mind: Suspension setup affects safety. Follow the vehicle maker's specifications and consult a qualified technician.

Frequently asked questions

How many lbf/in is 1 kgf/mm?

About 56.0 lbf/in.

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

Multiply by about 5.710.

What is a kilogram-force?

The force of gravity on 1 kg at standard gravity: exactly 9.80665 N.

Sources: NIST Special Publication 811 (2008 ed.), Appendix B.9 (kilogram-force, pound-force per inch, inch)

Checked 9 Nov 2026. NIST Special Publication 811 (2008 ed.), Guide for the Use of the International System of Units (SI), Appendix B.9 “Factors for units listed by kind of quantity or field of science” — kilogram-force = 9.806 65 E+00 N (exact), pound-force per inch = 1.751 268 E+02 N/m, inch = 2.54 E-02 m (exact).

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