How this calculator works
A linear spring rate is a force change divided by displacement change. Convert its force unit and inverse length unit separately. The kilogram-force is based on standard gravity, not on a local gravity measurement. A kip is 1000 pounds-force. Since a joule is a newton metre, J/m² has the same dimensions and numerical SI value as N/m. A torsional spring rate needs torque per angle and belongs to a different conversion.
Formula
1 lbf/in = 4.4482216152605/0.0254 N/m; 1 N/mm=1000 N/m; 1 kgf/mm=9806.65 N/m; 1 J/m²=1 N/m.
Worked example
A stiffness of 1 lbf/in is 175.126835 N/m. A stiffness of 1 N/mm is 1000 N/m, or 0.101971621 kgf/mm.
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Frequently asked questions
Does kg/mm mean kilograms-force per millimetre?
In many spring catalogues it does, but kg is formally a mass unit. This tool explicitly uses kgf/mm.
Is N m the same as N/m?
No. N m is a moment or energy unit; N/m is linear stiffness.
Can I enter a negative rate?
This converter accepts nonnegative ordinary spring rates. Negative incremental stiffness needs a model-specific interpretation.