I didn't see any posts for this so I figured I'd put it up in the name of using more safely. Volumetric dosing exploits the fact that when a solid (the solute) is dissolved in a liquid (the solvent), the solid automatically disperses evenly in the liquid (now called a solution). So if you have a gram of fentanyl and no milligram scale, how do you measure out a 1mg dose? There are 1000 milligrams in a gram, and 1000 CC's in a liter. So, dissolving a gram of fentanyl into one liter of distilled water means that 1cc of the solution will contain 1mg of fentanyl.
It's important to understand that dissolving is not the same as suspending. For example salt dissolves in water, but salt won't dissolve in isopropyl alcohol. Finely powdered salt will suspend in isopropyl, and on standing, the salt will eventually settle to the bottom. No matter how long salt water stands, the salt won't settle out. Before using volumetric dosing be sure your solid is compatible with your liquid. There are solvent tables all over the internet, and Google knows damn near everything.