How many ml are in 45mg?

Answer 1

The answer depends entirely on the substance you're converting. For example, 45 mg of lead might be only 10 ml, whereas 45 mg of water is 45 ml and 45 grams of hydrogen gas might be hundreds of ml. This is because metal is much more dense than water, which is more dense than gas. Note: the numbers used aren't accurate, but are to illustrate only.

Answer 2

Milligrams are units of mass or weight and milliliters are units of volume. They cannot be converted from one to the other.