The number of RBCs per cubic millimeter gives an indirect estimate of the Hb content of the blood. Manual blood cell counting chamber (hemocytometer) methods give errors of 7%-14% or even more, depending on the experience of the technician. Automatic counting machines reduce this error to about 4%. However, many smaller laboratories do not have these machines. Reference values are 4.5-6.0 million/mm3 (4.5-6.0 Ч 106/L) for men and 4.0-5.5 million/cu mm (4.0-5.5 Ч 106/L) for women.