Spring Compression Calculator | Find Spring Rate from Coil Dimensions
Enter wire diameter, coil outer diameter, free length, active coils, and material to compute the spring rate, maximum force, deflection, solid height, mean coil diameter, and spring index of a helical compression spring. Useful for first-pass design and for reverse-engineering an unmarked spring you already have.
💡 About this tool
Whether you are building a jig, tuning a 3D printer bed, repairing a piece of furniture, or prototyping a mechanism, you often end up holding a compression spring with no data sheet. Measure the wire diameter, outer diameter, and free length with calipers, count the active coils, and you can estimate its spring rate in seconds.
The spring rate (N/mm) is the force needed to compress the spring by one millimetre. This tool uses the standard mechanics-of-materials relation k = G·d⁴ / (8·D³·n), where d is wire diameter, D is mean coil diameter (outer diameter minus wire diameter), n is the number of active coils, and G is the shear modulus of the chosen material. It also reports solid height and the spring index so you can sanity-check whether the design is practical to manufacture.
🧐 Frequently Asked Questions
What does spring rate actually mean? It is the force, in newtons, required to compress the spring by one millimetre (N/mm). A higher rate means a stiffer spring that deflects less under the same load.
Why is max deflection capped at about 60% of free length? Compressing a spring until the coils bottom out (solid) can cause permanent set and early fatigue. About 60% of the free length is shown as a practical safe working deflection; adjust the margin for your own application.
What is the difference between active and total coils? Active coils are the ones that actually deflect. On a closed-end spring the end coil at each end does not contribute to deflection, so the solid-height estimate treats total coils as active coils plus two.
I got a spring index warning — what does it mean? The spring index is the mean coil diameter divided by the wire diameter. A very low index is hard to coil, and a very high index tangles easily. The tool flags values outside a manufacturable range, with 4 to 12 generally considered ideal.
Which material should I choose? Music wire (SWP) suits general-purpose springs, stainless (SUS304) is for corrosion resistance, and phosphor bronze (C5191) is used where conductivity or non-magnetic behaviour matters. Each has a different shear modulus, which changes the spring rate.
📚 How spring index shapes a design
Spring index is one of the most quietly important numbers in spring design. Manufacturers favour an index between roughly 4 and 12 because it balances cost, formability, and tolerance: a tight index below 4 puts heavy bending stress on the coiling tooling, while an index above 12 produces springs that tangle in bins and need extra handling. When you scale a design, keeping the index constant while changing wire and coil diameter together tends to preserve the spring's "feel," which is why catalogue families often share an index across many sizes. The simple rate formula here assumes a linear spring and ignores end-coil effects and stress, so treat the output as a starting estimate rather than a final stress-verified design.