A system for weighing a plurality of quadruped animals is for use in a pen confining the animals. The pen has at least first and second segregated spaces with a first one-way chute allowing animal passage from the second space to the first space. A weighing station is located within a passage having an entrance and an exit, and that allows animal passage from the first area to the second area. The weighing station includes a weighing platform within the passage over which an animal must pass when passing from the first to the second space. The weighing platform has inlet and exit scales that support the weighing platform adjacent to inlet and exit ends thereof, and provides inlet and exit weight signals. A controller receives the inlet and exit weight signals and uses them in an iterative process to determine the presence of a single animal on the weighing platform. Once that determination is made, the sum of the inlet and exit weight values is very likely to accurately provide the animal weight. The controller may also use the inlet and exit weight signals to control closing of an inlet gate to bar more than one animal at a time from the weighing platform. An animal sorting system can easily incorporate this weighing system to provide more accurate sorting.