Spring Index Calculator

Inputs

Known coil diameter

Choose whether the entered coil diameter is mean, outside or inside diameter.

Coil diameter (D)

Mean, outside or inside diameter according to the selector above.

Wire diameter (d)

Diameter of the spring wire.

Default result

Spring index(C)

Pass

Also computed

Mean coil diameter(D)10mm

Outside diameter(OD)11mm

Inside diameter(ID)9mm

Wahl factor(Kᵥᵥ)1.145

Method notes 3 notes

Spring index is C = D / d, mean coil diameter divided by wire diameter.
Low C values are hard to manufacture and raise stress concentration; high C values can buckle and tangle more easily.
Wahl factor is shown because it is the stress-correction term used by the spring calculators.