How many milligrams are in 5cc?

Answer 1

A milligram is a measure of mass. A cubic centimetre is a measure of volume. The two measure different things and, according to basic principles of dimensional analysis, conversion from one to the other is not valid.

If you are not convinced, consider a cc of air. How many grams? Next consider a cc of lead. How many grams?

Some people still believe that there is a conversion in relation to pure water but that is only approximately true. Until 1964 (more than 50 year ago!) a litre was defined as the volume of one kilogram of pure water at 4 degrees Celsius and at a pressure of 760 millilitres of Mercury. With that definition a conversion would have been valid - but only for pure water and only under those conditions. In any case that definition of a litre was abandoned in favour of 1 litre =1000 cubic centimetres. Under this definition, the specific gravity of pure water, at 4 deg C and 760 ml of mercury is not 1 but 0.999 972.