Whole blood units are usually around a pint or a bit over 450 mL Packed blood cells are usually around a half pint. According to the FAQ on the Gulf Coast Regional Blood Center web site, a unit is one pint, which equals about 473 mL. Approximately 450mL according to Canadian Blood Services. This depends on various factors and the volume of blood in each unit bag will not always be the same from one bag to another.