**To convert parts per million to milligrams per liter, use the conversion factor of 1 ppm = 1 mg/L.** This means that 1 part per million is equal to 1 milligram per liter.

Milligrams per liter is an important unit of measure that illustrates the concentration of dissolved substances. This unit of measure is a ratio of mass or weight per volume. This differs from parts per million, which instead is a measure of weight per weight or mass per mass.

The unit parts per million is used when there is a very low concentration of a solution. One gram of a solution in 1000 milliliters equals 1000 parts per million. When the mass of the solution is reduced to 1/1000 of a gram in 1000 milliliters, this equals 1 part per million. Because 1000 milliliters equals one liter, 1 part per million is 1 milligram per liter.

One part per million only equals 1 milligram per liter in the event that the solvent is water. This is a direct result of water's gravity, which is very close to one. When the solute, or the substance dissolved in the solution, is less than 1 percent, it can be assumed that the liter of water weighs 1,000 grams, which is equal to 1 million milligrams.