  A universal exponential factor, $\gamma_{c}=\pi/2$, is disclosed for the
dimensional crossover of few-layer graphene (FLG) from two-dimensional graphene
to three-dimensional graphite. $\gamma_{c}$ is found by analyzing available
experimental data on different properties of FLG with varying thickness. A
theoretical study on the phonon spectrum of the vertical acoustic mode in FLG
is carried out to further check this exponential factor $\gamma_{c}$.
Interestingly, the same exponential factor appears in the dimensional crossover
of the phonon mode. It turns out that the exponential factor $\gamma_{c}$ is
related to the homogeneous Helmholtz-like molal equation in the mass transfer
with a first order chemical reaction. The finding should provide valuable
information for experimentalists and theorists in the future investigation on
thickness dependent properties of FLG.
