  We present a set of formulae to extract the longitudinal deep inelastic
structure function $F_L$ from the transverse structure function $F_2$ and its
derivative $dF_2/dlnQ^2$ at small $x$. Our expressions are valid for any value
of $\delta$, being $x^{-\delta}$ the behavior of the parton densities at low
$x$. Using $F_2$ HERA data we obtain $F_L$ in the range $10^{-4} \leq x \leq
10^{-2}$ at $Q^2=20$ GeV$^2$. Some other applications of the formulae are
pointed out.
