The full grid on a hemocytometer contains 16 squares, each of which is 1 mm square. A dense suspension of small cells is 1/4 sq. mm.
Each large square has a surface area of 1.0mm2, and the depth of the chamber is 0.2mm. As there are 1000 mm3 per ml, each large square represents a volume of 0.0002ml, so that it is equal to 1/ 0.0002ml = 5,000. The volume factor is 5,000.