Inputs
Disclaimer
This calculator returns engineering estimates from standard, SMI-aligned spring formulas and idealized assumptions. Results are for preliminary design and RFQ preparation only, and should be verified by a qualified engineer before use in any critical application.
It does not account for every factor that affects real-world spring performance — including material lot variation, manufacturing tolerances, end-condition effects, operating temperature and environment, set and relaxation, or fatigue and cycle life.
Use at your own risk. Do not rely on these results where spring failure could cause injury, property damage, or loss of function. Always follow the applicable engineering standards, test for your specific application, and contact Tri-Matic Spring for a verified, production-ready specification.
Results
What the measurements mean
The geometry fields above map straight onto this drawing. Wire Diameter (d) is the thickness of the wire itself. Outer Diameter (OD) is measured across the outside of the coil, not across the wire. Free Length (L) is the length at rest, before any load is applied, and Total Coils (Nt) includes the closed end coils.
How this compression spring calculator works
A compression spring resists an axial load by twisting the wire along its length. Everything this tool reports comes from that single fact, using the standard closed-form equations published by the Spring Manufacturers Institute and in Shigley’s Mechanical Engineering Design. It is built for the moment before you send a drawing out — sanity-checking a concept, or working out whether a spring is even feasible in the space you have.
How do you calculate a compression spring’s rate?
k = G d⁴ / (8 D³ Na) k = rate (lbf/in) · G = shear modulus · d = wire diameter · D = mean diameter · Na = active coils
Rate scales with the fourth power of wire diameter and inversely with the cube of mean diameter. That is the most useful thing on this page: a ten percent increase in wire size raises rate by about 46 percent, while a ten percent increase in diameter drops it by roughly 25 percent. Small dimensional changes move load a long way, which is why coiled springs rarely hold the block tolerances printed on machined-part drawings.
Why is the stress corrected with the Wahl factor?
τ = Kw · 8 P D / (π d³) Kw = (4C−1)/(4C−4) + 0.615/C, the Wahl correction, where C = D/d
Wire on the inside of a coil is more highly stressed than the simple torsion formula predicts, because the wire is curved. The Wahl factor corrects for that, and it matters most at low spring index — at C = 4 it adds about 40 percent to the calculated stress. The calculator compares the corrected stress against an allowable derived from the material’s estimated tensile strength (Sut = A/dm), which is itself a function of wire size: thinner wire is stronger per unit area.
What does each input control?
- Wire diameter and outside diameter — mean diameter is OD minus one wire diameter. Spring index C = D/d falls out of these two, and index between 4 and 12 is the practical coiling range. Below 4 the tooling struggles and stress concentrates; above 12 the spring tangles and buckles.
- Total coils and end type — active coils are what actually deflect. Closed ends consume roughly two coils, so a ten-coil closed-and-ground spring has eight active. End type also sets solid height, which is why it changes both rate and available travel.
- Free length and working deflection — together these give travel to solid and the load at your working point. If deflection exceeds travel to solid, the calculator flags it rather than silently reporting a load the spring cannot reach.
When does this estimate stop being reliable?
These are static, idealised results. The calculator does not model buckling in long springs, fatigue and cycle life, set and relaxation under sustained load, elevated or cryogenic temperature, dynamic surge at high cycle rates, or the tolerance stack of a real production run. Any of those can dominate in service. Treat the numbers as a starting point and send us the drawing for a specification we will stand behind.
What units does the calculator use?
Switch the unit selector and every field and result converts: inches with lbf, lbf/in and psi, or millimetres with N, N/mm and MPa. Deflection angles, where they appear, stay in degrees in both systems.