Spring Wire Size Calculator

Inputs

Target spring rate (k)

Required stiffness of the spring.

N/mm

Mean coil diameter (D)

Center-to-center coil diameter. Outside diameter will be D + wire diameter.

Active coils (Nₐ)

Coils that actually deflect under load.

Free length (L₀)

Unloaded length used for solid height and travel checks.

Working deflection (x)

Deflection used to calculate force and stress after the wire size is solved.

Material

Sets shear modulus and allowable stress.

End type

Sets inactive end coils and solid height.

Default result

Required wire diameter(d)

1mm

Pass

Also computed

Outside diameter(OD)11mm

Inside diameter(ID)9mm

Spring index(C)10

Force at deflection(F)12.39N

Corrected shear stress(τ)361.2MPa

Allowable shear stress995MPa

Method notes 3 notes

Wire diameter is solved from d = (8·k·D³·Nₐ / G)^(1/4).
Because rate depends on d⁴, small wire-diameter changes strongly affect spring rate.
Check stress, spring index and travel-to-solid before treating the wire size as usable.