The Practical Salinity Scale defines salinity in terms of the conductivity ratio of a sample to that of a solution of 32.4356 g of KCl at 15°C in a 1 kg solution. A sample of seawater at 15°C with a conductivity equal to this KCl solution has a salinity of exactly 35 practical salinity units (psu).

In practice, salinity is determined from empirical relationships between temperature and the conductivity ratio of a sample to International Association for the Physical Sciences of the Ocean (IAPSO) Standard Seawater. Comparison of results with other laboratories requires all researchers to use the IAPSO Standard Seawater for calibration.