Spring rate describes how much force changes for a corresponding change in deflection. In a linear region it is the slope of the load–deflection curve.
Calculate rate from two points
Use consistent force and deflection units. The result inherits the selected units.
The basic formula
k is spring rate, F is force and x is deflection. If force is measured in newtons and deflection in millimetres, the rate is N/mm. With kilogram-force and millimetres, it is kgf/mm.
Worked example
A spring measures 40 N at 10 mm and 100 N at 25 mm:
Within that interval, each additional millimetre of movement requires approximately 4 N of additional force.
Why the interval matters
The first part of a curve may include seating and the final part may become nonlinear. A defensible recipe defines the rate interval in advance—for example, between 20% and 80% of the test travel or between two drawing-defined points. Using the same interval across samples makes the comparison meaningful.
For noisy data, a fitted slope across the selected region can be more stable than using only two readings. The report should identify the calculation range so another reviewer can reproduce it.
Compression and extension
The same slope concept applies to compression and extension. Extension springs may also have initial tension, so extrapolating the useful linear region toward zero extension can produce a non-zero force intercept. That intercept and the spring rate are different characteristics.
Unit discipline
Keep force and distance units together. Convert the complete recipe and results consistently rather than converting labels alone. Common rate units include N/mm, kgf/mm and lbf/in. A value without its unit and calculation interval is incomplete.
This guide provides general engineering information. Apply the governing drawing, licensed standard, risk assessment and approved procedure for the actual test.