Concentrations may be noted in equations as ug/mL, or micrograms per milliliter. One gram equals 1 million micrograms. Concentration also may be listed as parts per million. It is used to conveniently express very small concentrations of a compounds, such as contaminates in water. When converting concentration, take into account the density of water depends upon temperature.

Find out the density of the water at the given temperature (see Resources). For instance, at 20 degrees Celsius, density is 998.2 kg/m^3.

Divide the water density by 1,000 to convert it from kg/m^3 to g/mL, In the example, 998.2 / 1,000 = 0.9982 g/mL.

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Multiply the density by 1 mL to calculate the weight of water. Continuing the example, weight is 0.9982 x 1 = 0.9982 g.

If using microgram units, divide the concentration by 1,000,000 to convert it to grams. For example, if the concentration is 16 ug/mL, then 16 /1,000,000 = 0.000016 g/mL.

Substitute the mL units by the weight of water calculated in step 3. In this example, 0.000016 g/ 1 ml = 0.000016 g/ 0.9982 g = 0.000016 / 0.9982.

Divide the numerator of the fraction by its denominator to present the number as a new fraction with the denominator 1. In this example, 0.000016 / 0.9982 = 0.00001603 / 1.

Multiply both the numerator and denominator of the new fraction by 1,000,000 to calculate parts per million. In the example, (0.00001603 / 1 ) x 1,000,000 = (0.00001603 x 1,000,000 ) / (1 x 1,000,000 ) = 16.03 / 1,000,000 = 16.03 ppm since the denominator of the fraction is 1 million.