  The size of the smallest $k$-regular graph of girth $g$ is denoted by the
well studied function $n(k,g)$. We suggest generalizing this function to
$n(H,g)$, defined as the smallest size girth $g$ graph covering the, possibly
non-regular, graph $H$. We prove that the two main combinatorial bounds on
$n(k,g)$, the Moore lower bound and the Erd\"{o}s Sachs upper bound, carry over
to the new setting of lifts, even in their non-asymptotic form.
  We also consider two other generalizations of $n(k,g)$: i) The smallest size
girth $g$ graph sharing a universal cover with $H$. We prove that it is the
same as $n(H,g)$ up to a multiplicative constant. ii) The smallest size girth
$g$ graph with a prescribed degree distribution. We discuss this known
generalization and argue that the new suggested definitions are superior.
  We conclude with experimental results for a specific base graph and with some
conjectures and open problems.
