Spring Index

Spring index, abbreviated as C, a dimensionless number, is the ratio between the spring and wire diameter for a helical spring.  This number tells how tight the springs coils are.  All wire sizes have a limit to the coil diameter being formed, too small there can be damage to the material.  A higher spring index has a smaller wire and a larger spring diameter.  A lower spring index has a larger wire and a smaller spring diameter.

 

Spring Index formulas
\(\large{ C = \frac{ D }{ d }  }\)  \(\large{ C = \frac{ 2 \; r }{ d }  }\)
Symbol	English	Metric
\(\large{ C }\) = spring index	\(\large{ dimensionless }\)
\(\large{ D }\) = spring diameter	\(\large{ in }\)	\(\large{ mm }\)
\(\large{ r }\) = spring radius	\(\large{ in }\)	\(\large{ mm }\)
\(\large{ d }\) = wire diameter	\(\large{ in }\)	\(\large{ mm }\)