# Strain

$\epsilon = {{l_i - l_0} \over l_0} = {\Delta l \over l_0}$
where $l_0$ is the original length before any load is applied, and $l_i$ is the instantaneous length. $\Delta l$ is the deformation elongation, or change in length at some instant, as referenced to the original length. This strain is typically referred to as "engineering" strain, and it a unitless quantity. Oftentimes strain is expressed as a percentage, so the strain value is simply multiplied by 100.