A history lesson
Since blood cells are counted per unit volume (per liter), it is vital that the volume of blood, in which the cells are counted, corresponds to a known quantity. This makes the Neubauer counting chamber a useful method.
The counting grid is composed of 9 big squares, measuring 1 x 1 mm . From these squares, the central square contains 25 medium sized squares each measuring 0.2 x 0.2 mm. These are further divided into 16 small squares each measuring 0.05 x 0.05 mm. The large central square is also called the erythrocyte grid. The squares highlighted in red correspond to 80 small squares, that are used to establish the erythrocyte and platelet counts. The large squares marked in blue are used to establish the leukocyte count.
Manual Cell Counting (by microscope)