  We provide a consistent framework for estimating galaxy counts and variances
in wide-field images for a range of photometric bands. The variances include
both Poissonian noise and variations due to large scale structure. We
demonstrate that our statistical theory is consistent with the counts in the
deepest multiband surveys available. The statistical estimates depend on
several observational parameters (e.g. seeing, signal to noise ratio), and
include a sophisticated treatment of detection completeness. The JAVA
calculator is freely available and offers the user the option to adopt our
consistent framework or a different scheme. We also provide a summary table of
statistical measures in the different bands for a range of different fields of
view.
  Reliable estimation of the background counts has profound consequences in
many areas of observational astronomy. We provide two such examples. One is
from a recent study of the Sculptor galaxy NGC 300 where stellar photometry has
been used to demonstrate that the outer disc extends to 10 effective radii, far
beyond what was thought possible for a normal low-luminosity spiral. We confirm
this finding by a reanalysis of the background counts. Secondly, we determine
the luminosity function of the galaxy cluster Abell 2734, both through
spectroscopically determined cluster membership, and through statistical
subtraction of the background galaxies using the calculator and offset fields.
We demonstrate very good agreement, suggesting that expensive spectroscopic
follow-up, or off-source observations, may often be bypassed via determination
of the galaxy background with GalaxyCount.
