To convert a milligram into milliliters, you must first know the density, or mass/volume ratio. Convert mass in milligrams to the unit of mass expressed in the density, then divide mass by density to convert to volume units. If the volume unit is not in milliliters, convert it to milliliters.
- Look up the density of the substance
Density of a substance, a mass/volume ratio, is commonly expressed in units like g/ml, g/cm^3, or kg/m^3. When you look up the density of the substance, be sure to note the units being used.
- Convert milligrams to the mass unit used in density
Unless density is in milligrams per unit of volume, a mass unit conversion of milligrams is required. For example, if density is in g/cm^3, you must convert milligrams to grams. Since 1,000 mg equals 1 gram by definition, 1 mg = 1/1,000 grams, or 0.001 grams.
- Divide mass by the density to convert to volume
Divide the mass, having the similar mass unit as the density, by the density value. Cancel out units when doing the calculation to come up with the unit of volume. For example, if the mass is 0.001 grams and the density is 0.917 g/cm^3, then 0.001 g / (0.917 g/cm^3) = 0.00109 cm^3. During the calculation, grams divided by grams cancels out, and you are left with cm^3.
- Convert the unit of volume to ml
If dividing by density yields a different unit of volume than ml, convert volume to ml. For example, if you have 0.00109 cm^3 as volume, the volume in ml is 0.00109 ml, because 1 cm^3 = 1 ml. Whereas, if your volume was in m^3, multiply the number of m^3 by 1 million to get the number of ml because 1 m^3 = 1,000,000 ml.