What is spring rate — and why does it matter?

Spring rate is one of the most important parameters when selecting a spring — yet it's one of the most commonly misunderstood. Whether you're designing a machine, specifying a component for production, or replacing a spring in an existing assembly, understanding spring rate will save you time, money and failed prototypes.

What is spring rate?

Spring rate — also written as R or k — is the force required to compress or extend a spring by one millimetre. It is expressed in Newtons per millimetre (N/mm).

A spring with a rate of 10 N/mm requires 10 N of force to compress it by 1 mm, 20 N to compress it by 2 mm, and so on. This linear relationship holds throughout the spring's working range.

Put simply: a high spring rate means a stiff spring. A low spring rate means a soft spring.

Why spring rate matters

Choosing the wrong spring rate has real consequences:

Too stiff: The spring doesn't compress enough under load, meaning the mechanism doesn't move as intended — or the force required is too high for the actuator or operator.

Too soft: The spring compresses too easily, potentially bottoming out (reaching solid height) under load. This can cause shock loads, noise, and premature failure.

Getting the rate right means your spring delivers the correct force at the correct deflection — every time.

How spring rate is calculated

For a helical compression spring, the spring rate is calculated using this formula:

R = (G × d⁴) / (8 × Dm³ × n)

Where:

• G = shear modulus of the material (N/mm²) — approximately 81,500 N/mm² for carbon steel, 73,000 N/mm² for stainless steel
• d = wire diameter (mm)
• Dm = mean coil diameter (mm) — outer diameter minus wire diameter
• n = number of active coils

The formula shows why wire diameter has such a large effect on spring rate — it appears to the fourth power. Doubling the wire diameter increases the spring rate by a factor of 16.

A practical example

A compression spring with:

• Wire diameter d = 1.2 mm
• Outer diameter Do = 12 mm → mean diameter Dm = 10.8 mm
• Active coils n = 8
• Material: carbon steel (G = 81,500 N/mm²)

Spring rate R = (81,500 × 1.2⁴) / (8 × 10.8³ × 8) = (81,500 × 2.0736) / (8 × 1,259.7 × 8) ≈ 1.67 N/mm

How to find a spring with the right rate

If you know the force you need to apply and the deflection you need to achieve, divide one by the other:

R = Force (N) ÷ Deflection (mm)

For example, if you need 25 N of force at 15 mm of compression: R = 25 ÷ 15 = 1.67 N/mm.

You can then use the Spring Calculator to verify the rate of any spring in our catalogue, or search for springs that match your target rate using the filter on the Compression Springs page.

What if you can't find an exact match?

Standard catalogue springs are manufactured to defined tolerances — a small variation in spring rate is normal and usually acceptable. If your application requires a tighter rate tolerance or a specific rate not available in our catalogue, contact us — we can source or produce springs to your specification.

Not sure where to start? Ask Springy — our AI assistant in the bottom right corner — and describe your application. It can help you work out the spring rate you need and find a suitable spring in our catalogue.

---

Key takeaways

• Spring rate (R) = force per mm of deflection, expressed in N/mm
• Higher rate = stiffer spring; lower rate = softer spring
• Rate is strongly influenced by wire diameter (to the fourth power) and number of active coils
• Use the Spring Calculator to verify or calculate spring rate for any spring
• If you need a specific rate not in our catalogue, we can help