Tracer tests are important for understanding transport in bedrock aquifers because they directly measured how rapidly groundwater moves. Thus they do not depend on assumptions about the presence or absence of preferential flow, or on an estimated value of effective porosity. Assessing the mass of tracer to inject has often been a challenge, but this paper used results from 211 natural-gradient sinking stream to spring tests and 44 forced-gradient well to well tests in carbonate aquifers to derive a series of empirical equations relating tracer mass to time, distance, and discharge. The figures show two of the regression equations that help determine the mass of tracer to inject.