Spring Index Calculator
SI
Imp
Imperial
Inputs
View results
Known coil diameter
Choose whether the entered coil diameter is mean, outside or inside diameter.
Mean diameter
Outside diameter
Inside diameter
Coil diameter
(D)
Mean, outside or inside diameter according to the selector above.
mm
Wire diameter
(d)
Diameter of the spring wire.
mm
Results
Default result
Edit inputs
Spring index
(C)
10
Pass
Also computed
Mean coil diameter
(D)
10
mm
Outside diameter
(OD)
11
mm
Inside diameter
(ID)
Pass
9
mm
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.
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