  The most accurate neutron lifetime measurements now use the material or
magnetic traps of ultracold neutrons (UCN). The precision of these experiments
is determined by the accuracy of estimating the neutron loss rate. In material
UCN traps the main source of neutron losses is the absorption by trap walls. In
this paper we analyze the standard methods and their approximations for the
calculation of UCN absorption rate by the walls of material traps. We emphasize
the approximations used both in the standard analytical formulas and in the
numerical Monte-Carlo simulations. For the two simplest trap geometries,
rectangular and cylindrical, we obtain precise analytical formulas for this
absorption rate and compare them with the standard estimation methods. The
difference turned out to be considerable and especially important for the size
extrapolation procedure, always used in the standard estimates of UCN losses.
Our results may partially resolve the puzzling four-second discrepancy between
the magnetic and material-trap measurements of neutron lifetime.
