Define your variables. You want to find how much mass of a substance there is per liter volume of liquid. So, you need to know how much liquid you have and how much of the substance to set up your equation. As an example, use a 6-liter tank and 5 grams of salt.
Do any necessary conversions to get your correct units. Use a calculator based on known constants. For example, if the volume were in gallons, one gallon is equal to about 3.79 liters and one gram is equal to 1,000 milligrams. In the example above, the volume is already in liters, but the 5 grams will have to be converted into 5,000 milligrams.
Set up your equation. To find the mg/L, divide the number of milligrams by the number of liters. For the 6-liter tank example, the equation would look like this: 5,000 mg / 6 liters.
Solve the equation with the calculator. For the example, you should have found that there are 833.33 mg/L or 833.33 milligrams of salt for every one liter of water in the tank. If you want to convert your findings back to grams, divide your answer by 1,000. You will find that in the example there are .83 grams of salt for every one liter of water in the tank.