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, initial tension and hook/end stresses, 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 body. Free Length Inside Hooks (L0) is measured hook to hook at rest, which is why it is longer than the coiled body alone.
How this extension spring calculator works
An extension spring differs from a compression spring in two ways that matter more than anything else: it is wound with initial tension, so it resists before it moves at all, and it carries hooks, which are usually the first thing to fail. This calculator handles both explicitly, using the standard SMI and Shigley relationships.
Why does the load include initial tension?
P = Ti + k y k = G d⁴ / (8 D³ Nb) Ti = initial tension · y = deflection · Nb = body coils (all of them are active)
At zero deflection the load equals the initial tension, not zero. Forget that and every load figure is wrong by a constant. Unlike a compression spring, every body coil is active — there are no dead end coils — so rate comes straight from the body count.
Why does hook stress usually govern the design?
σ = P [ KA · 16D/(πd³) + 4/(πd²) ] bending at the loop transition, with KA = (4C²−C−1)/(4C(C−1))
The calculator reports body stress and hook stress separately, and the hook figure is normally the higher of the two. Extension springs overwhelmingly fail at the hook, where the wire is bent through a tight radius and sees bending rather than pure torsion. If you are designing for cycle life, design to the hook number. Because real hook geometry varies, treat that value as indicative and let us review the actual bend radius.
Should you use geometry mode or load mode?
- Geometry mode — you know the wire, diameter, body coils and initial tension. Everything else is derived. Use this when you have a design and want to check it.
- Load mode — you know the load you need at a given extended length. The calculator estimates initial tension from spring index, the standard industry method, then derives rate and body coils from your load point. Initial tension is set on the coiler during manufacture, so a load-mode result is a target to quote against, not a finished spec.
How is free length inside the hooks estimated?
Free length inside the hooks is estimated as the close-wound body plus one inside diameter per hook, which is the standard machine-hook proportion. Extended hooks, side loops, crossover hooks and other non-standard ends change that number, sometimes substantially. Estimated weight covers body wire only; the hooks add material.
When does this estimate stop being reliable?
Static and idealised. Not modelled: fatigue and cycle life, stress relaxation under sustained extension, temperature effects, hook geometry beyond the standard assumption, or production tolerance. Extension springs are also frequently over-extended in service, which sets them permanently — a failure mode no formula predicts. Send us the application and we will spec it properly.
What units does the calculator use?
Inches with lbf, lbf/in and psi, or millimetres with N, N/mm and MPa. Switch the selector and every field converts.