AbstractThe equation to calculate the retardation factor of sorbing chemicals moving in aqueous solution through aquifers and vadose zones is derived using a linear sorption isotherm and a mass‐balance approach. The procedure is very simple and clearly illustrates the roles of porosity and water content, including those for saturated flow, unsaturated flow, and preferential flow or other systems with both mobile and immobile phases of the water. Applications of the equation to preferential flow in the vadose zone and macrodispersion in a layered aquifer are illustrated with numerical examples for hypothetical case