Groundwater density is an important parameter in the interpretation of flow patterns. This paper investigates the relationship between electrical conductivity (EC) and groundwater density in coastal aquifers and evaluates the suitability of the UNESCO 1980 equation of state, developed for the world's oceans, for determining the density of groundwater based on its EC. To achieve this aim, a dataset of groundwater samples from four different types of coastal aquifers was collected. It is found that the density of a sample can be estimated to a good approximation on the basis of its EC using the UNESCO 1980 equation of state. Deviations from the equation of state were found to be due to the changes in EC and the density caused by geochemical reactions, such as the dissolution of carbonates, degradation of organic carbon, cation exchange, and sulfate loss. Owing to these deviations, the UNESCO 1980 equation of state may underestimate the density by up to 1.5 kg/m3. The effect of this uncertainty on the correction terms applied to the hydraulic head required for a proper interpretation of groundwater flow patterns and rates is quantified. It was found that the fresh water head may be wrong by centimeters to a few decimeters. From this it is concluded that, unless the purpose of a groundwater investigation requires great accuracy, the equation of state provides an efficient and inexpensive way to estimate density from EC.