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.
Milligrams are units of mass or weight and milliliters are units of volume. They cannot be converted from one to the other.
45mg
There are 45mg of sodium in an 8 oz serving of Sprite.
This cannot be sensibly answered. A milliliter (mL or ml) is a measure of volume, grams (g), kilograms (kg) and milligrams (mg) are measures of weight or mass.
45mg is less than 5g.
typical frozen wild salmon from your local mega mart has: 100 calories/4oz serving 2.5 fat grams 45mg cholesterol
That is 3.75 ml.
1900 ml
45 mg contains approximately 1.956 moles of atomic sodium.
That is 4,250 ml.
That is 600,000 ml.
That is 12,000 ml.
That is 4,000,000 ml.