125 milligrams equals how many milliliters?

Milligrams are a measure of weight and milliliters are a measure of volume and thereforeit is not feasible to convert one to the other.

It is true to say that 125 milligrams of pure water occupies 125 millilitres of volume at a certain temperature and barometric pressure but this will not apply to all materials and nor does it appy to pure water at every temperature and pressure.