mL (millitre) is a volume, mcg (microgram or µg) is a weight.
If this is for medicine, you have a small amount of drug dissolved in a much larger volume of liquid.
If you are trying to find the volume of a weight of liquid, you need to know the density. However that is a very small volume for that weight, and would need to be measured in a micro syringe. - You would hardly be able to see it.