  We describe an algorithm to detect voids in galaxy redshift surveys. The
method is based on the void finder algorithm of El-Ad & Piran. We apply a
series of tests to determine how accurately we are able to recover the volumes
of voids using our detection method. We simulate voids of different ellipticity
and find that if voids are approximately spherical, our algorithm will recover
100% of the volume of the void. The more elliptical the void, the smaller the
fraction of the volume we can recover. We insist that voids lie completely
within the survey. Voids close to the edge of the survey will therefore be
underestimated in volume. By considering a deeper sample, we estimate the
maximal sphere diameters are correct to within 30%.
  We apply the algorithm to the Point Source Catalogue Survey (PSCz) and the
Updated Zwicky Catalog (UZC). The PSCz survey is an almost all-sky survey with
objects selected from the IRAS catalog. The UZC covers a smaller area of sky
but is optically selected and samples the structures more densely. We detect 35
voids in the PSCz and 19 voids in the UZC with diameter larger than
20h^{-1}Mpc. Using this minimum size threshold, voids have an average effective
diameter of 29.8+-3.5 h^{-1}Mpc (PSCz) and 29.2+-2.7 h^{-1}Mpc (UZC) and that
they are underdense regions with delta rho /rho values of -0.92+-0.03 (PSCz)
and -0.96+-0.01(UZC) respectively. Using this quite stringent threshold for
void definition, voids fill up to 40% of the volume of the universe.
