Chemists have a wide variety of units at their disposal with which to describe the concentration of chemicals in mixtures. One unit is percent by weight which is the grams of chemical per one hundred grams total mixture. More dilute mixtures are sometimes described in terms of milligrams chemical per kilogram of total mixture weight; this is abbreviated as "mg/kg." Since there are 1 million milligrams in a kilogram, these units of mg/kg are identical to units of parts per million. You can convert between percent and mg/kg units by a single conversion factor based on the ratio of 100 to 1 million.
Enter the numerical value of your mixture's percent by weight concentration into the calculator. For example, if you had a mixture of chemicals which was 0.035 percent sodium chloride, you would enter 0.035.
Multiply the value you just entered by 10,000. This is the conversion factor between percent by weight and mg/kg, or ppm. The reason that 10,000 is used is that this is the ratio between 100 — in the percentage units — and 1 million — in the ppm units.
Report the result of this calculation as the concentration of the chemical in your mixture in units of mg/kg. In the case of the example, you would calculate 0.035 times 10,000 and report the concentration as 350 mg NaCl/kg.
Since one liter of water at room temperature weighs almost exactly one kilogram, the concentration of a dilute water solution in mg/kg is almost identical to its concentration in units of mg/L.