  We determine the relative pion mass shift $M_\pi(L)/M_\pi-1$ due to the
finite spatial extent $L$ of the box by means of two-flavor chiral perturbation
theory and the one-particle L\"uscher formula. We use as input the expression
for the infinite volume $\pi\pi$ forward scattering amplitude up to
next-to-next-to-leading order and can therefore control the convergence of the
chiral series. A comparison to the full leading order chiral expression for the
pion mass in finite volume allows us to check the size of subleading terms in
the large-$L$ expansion.
