How many drops in 140mg?

Answer 1

There can be no equivalence.

A milligram is a measure of mass. A drop 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.

Furthermore, a drop is not a standard size: it depends on the viscosity of the fluid. Also, you may have noticed that in sometimes the drops of water in rain are much larger than at other times.

In any case, consider a drop of air. How many milligrams? Next consider a drop of syrup. How many milligrams?

The masses of equal volumes of the two substances will clearly be very different. So there is no direct conversion between mass and volume: you need to know the density of the substance to enable you to carry out the conversion.