  Hecke-Kiselman algebras $A_{\Theta}$, over a field $k$, associated to finite
oriented graphs $\Theta$ are considered. It has been known that every such
algebra is an automaton algebra in the sense of Ufranovskii. In particular, its
Gelfand-Kirillov dimension is an integer if it is finite. In this paper, a
numerical invariant of the graph $\Theta$ that determines the dimension of
$A_{\Theta}$ is found. Namely, we prove that the Gelfand-Kirillov dimension of
$A_{\Theta}$ is the sum of the number of cyclic subgraphs of $\Theta$ and the
number of oriented paths of a special type in the graph, each counted certain
specific number of times.
