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Standard Methods uses the International System of Units (SI).1 Quantities of mass and volume resulting from measurements and analyses are used to calculate and report concentrations of chemical and physical constituents in water. Concentration values must be reported using a unit of expression that is appropriate for the intended use of the data and within an appropriate value range for the unit expression used. For example, a typical unit expression for concentration is milligrams per liter (mg/L) where numerical values range from 0.1 to 999.9 mg/L. If the concentration value is less than 0.1, it is customary to express the concentration as micrograms per liter (μg/L) and if greater than 999.9, then express as grams per liter (g/L), with the numerical values adjusted accordingly. Record and report all measured and analytical results to the proper number of significant figures (see 1050 B).