  It is proved that if $G=G_1*_{G_3}G_2$ is free product of probability measure
preserving $s$-regular ergodic discrete groupoids amalgamated over an amenable
subgroupoid $G_3$, then the sofic dimension $s(G)$ satisfies the equality \[
s(G)=\h(G_1^0)s(G_1)+\h(G_2^0)s(G_2)-\h(G_3^0)s(G_3) \] where $\h$ is the
normalized Haar measure on $G$.
