  We provide an estimate on the absolute values of the emission rate of photon
pairs produced by spontaneous parametric down conversion in a bulk crystal when
all interacting fields are in single transverse Gaussian modes. Both collinear
and non-collinear configurations are covered, and we arrive at a fully
analytical expression for the collinear case. Our results agree reasonably well
with values found in typical experiments, which allows this model to be used
for understanding the dependency on the relevant experimental parameters.
