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.
45mg
45 mg contains approximately 1.956 moles of atomic sodium.
45mg is less than 5g.
1900 ml
That is 3.75 ml.
typical frozen wild salmon from your local mega mart has: 100 calories/4oz serving 2.5 fat grams 45mg cholesterol
29.57 ml
5
10
That is 9,000 ml
That is 4,250 ml.
That is 47,000 ml,