What Is Spring Rate and How to Calculate It | AxiSpring
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Spring rate is the single most important parameter when selecting a spring - yet it is often misunderstood or overlooked. This article explains what spring rate means, how to calculate it, and how to use it in practice when specifying a spring for your application.
What is spring rate?
Spring rate (also called spring constant or spring stiffness) is the amount of force required to compress or extend a spring by one unit of length. It is expressed in Newtons per millimetre (N/mm) and given the symbol R or k.
A spring with a rate of 5 N/mm requires 5 N to move 1 mm, 50 N to move 10 mm, and 500 N to move 100 mm. The relationship is linear for standard coil springs - this is known as Hooke's Law:
F = R × s
Where F is force in Newtons, R is spring rate in N/mm, and s is deflection in mm.
Why spring rate matters
Spring rate determines how your spring behaves under load. Choose too low a rate and the spring will compress fully under normal loads, leaving no travel left. Choose too high a rate and the spring will barely move, providing little cushioning or travel.
Getting the rate right means your spring will operate within its designed deflection range - protecting both the spring and the equipment around it.
How to calculate the spring rate you need
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The force you need at a specific deflection point (in N)
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The deflection at that point (in mm)
Then divide: R = F ÷ s
Example: You need a spring to exert 80 N when compressed 16 mm from its free length.
R = 80 ÷ 16 = 5 N/mm
Now search for a compression spring with a spring rate of 5 N/mm and a free length long enough to allow 16 mm of compression in your assembly.
Operating range and safety margin
Never design a spring to work at its solid length - the point where all coils are fully touching. Beyond 80% of maximum deflection, stress increases sharply and fatigue life drops.
A practical rule: design your operating point at 60-75% of maximum deflection. This leaves room for overloads and extends spring life significantly.
Spring rate for different spring types
Compression and extension springs - rate is constant across the operating range (linear). Expressed in N/mm.
Torsion springs - rate is expressed in Nmm per degree (Nmm/°), since the spring rotates rather than translates.
Gas springs - force is not perfectly linear; it increases slightly as the spring is compressed due to gas compression. The rated F1 force is measured at full extension.
Disc and wave springs - rate depends on stacking configuration. Springs in series reduce the effective rate; springs in parallel increase it.
Finding springs by rate on AxiSpring
Every product in our catalog includes spring rate in the filter table and on the product page spec sheet. Use the rate filter to narrow down to springs within your required range - then check free length and diameter to complete your selection.