# Spring Index

Written by Jerry Ratzlaff on . Posted in Fastener

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

 $$\large{ C = \frac{ D }{ d } }$$ $$\large{ C = \frac{ 2 \; r }{ d } }$$

### Where:

 Units 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 $$\large{ in }$$ $$\large{ mm }$$

Tags: Equations for Spring