the hemocytometer is 0.1 mm deep and the 25 large squares represent an area of 1 square mm.the volume above the 25 squares shown in 0.1 nm. As you are only counting 5 squares,you counted the sperm that settled out of 0.02 nm(0.1/5=0.02). therefore you must multiply your count in 5 squares by 50000 in order to determine how many sperm would have been in 1.0 ml (1000/0.02=50000; we usually express sperm concentration in terms of numbers/ml). To get the concentration of the original sperm sample we must however also multiply by the dilution factor. The equation that follows would be used to convert the count in 5 squares to concentration/ml