Where;

**F** is the force applied to the compression spring (lbs)

**k** is the spring rate, often specified in catalogs. (lbs/inch) Also, see below for calculation of spring rate.

**L**_{f} is the length of the spring with zero deflection, ie, no force applied. Often called "free length" (inches).

**L**_{d} is the length of the spring with the force F applied. (inches).

There is a comprehensive compression spring calculator/designer here at efunda.