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.
- Set rounding increments. Use the wire-size and active-coil increments your supplier or shop can actually build.
- Review exact and rounded results. Read the exact wire/coil solution, then check the rounded candidate load error.
- Check stress and travel. Use the exact and rounded stress/travel verdicts before sending a candidate for manufacturing review.
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.
The exact answer is rarely the final shop answer, so the calculator also builds a rounded spring candidate. It rounds wire diameter to the entered wire increment and active coils to the entered coil increment, then recalculates spring rate, load at the working deflection, load error, stress utilization and travel margin for that manufacturable candidate.
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.
If the coil count is rounded to the nearest half coil, the candidate becomes
9.5 active coils, carries about 50.9 N at the same
travel and lands about 1.9% above the target load.
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.
Why is there a rounded spring candidate?
The exact wire diameter and active coil count are useful for math, but a spring shop needs manufacturable values. Enter a wire-size increment and active-coil increment, then the calculator rounds the design and re-checks load, stress and travel margin.
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.
- Rounded candidate checks use user-entered increments, not a supplier stock-wire table.
- 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.