How to use this calculator
- Enter the required load and travel. Use the working force and deflection at the operating point.
- Choose the coil diameter and spring index. Pick a mean diameter that fits your envelope and a target index, often 6 to 10.
- Set free length and material. Free length controls travel to solid, while the material sets shear modulus and allowable stress.
- Review wire and coil count. Read the solved wire diameter and active coil count.
- Check stress and travel. Use the stress and travel margin verdicts before rounding the design for manufacturing.
How it works
This calculator works backward from the load point. First it computes the required
spring rate:
k = F / x
where F is the target load and x is the working deflection.
You choose the mean coil diameter D and target spring index
C = D/d, so the wire diameter is d = D/C. The active
coil count is then solved from the helical spring rate equation:
Na = G*d^4/(8*D^3*k).
The solved spring is then checked like a normal compression spring: Wahl-corrected shear stress, static allowable stress, solid height, inside/outside diameter and travel margin before solid height.
Worked example
Verified against the live calculator
Suppose the spring must carry 50 N after 10 mm of
compression. The required rate is k = 50 / 10 = 5 N/mm.
With music wire, mean diameter D = 20 mm and target index
C = 8, the wire diameter is d = 20 / 8 = 2.5 mm.
The rate equation gives Na = 9.68 active coils. Closed and ground
ends make that about 11.68 total coils, with 29.2 mm
solid height and about 20.8 mm of travel margin at the load point.
Frequently asked questions
How do you design a compression spring from load and travel?
Start with the required rate k = F / x. Choose a mean coil diameter and spring index C = D/d, which sets wire diameter d = D/C. Then solve active coils from Na = G*d^4/(8*D^3*k).
What spring index should I target?
A practical first target is usually C = 6 to 10, with about 4 to 12 as the broader manufacturable range. Lower values are hard to wind and raise stress; higher values can tangle or buckle.
Why does the calculator solve active coils instead of total coils?
The rate equation uses active coils, because only active coils deflect. Total coils depend on the end style: closed and ground ends add inactive coils, while open ends use nearly all coils as active.
What should I do after getting a design?
Round wire diameter and coil count to values your spring shop can make, then re-check the rounded design with the compression spring calculator for rate, stress, solid height and buckling.
Does this check fatigue life?
No. This is a static first-pass design check. Fatigue life needs stress range, mean stress, surface condition, shot peening, set removal and the expected cycle count.
Does this work in metric and imperial?
Yes. Toggle SI/Imperial in the header; the physics runs in fixed internal units and the inputs/results are converted for display.
Method & assumptions
- Round-wire helical compression spring with linear elastic behavior.
- Target spring index is user-selected; the calculator does not optimize across stock wire sizes.
- Stress uses the Wahl correction factor and a static material allowable.
- Fatigue, set removal, shot peening, surge, tolerances and manufacturer process limits are separate checks.