The difference between purified water and distilled water lies in the production method, the degree of purity, and the applications. Purified water is produced specifically for critical applications, such as those in the pharmaceutical and biotechnology industries, and must meet strict quality standards.

Quality is often measured by the water's resistance, which at 25 °C corresponds to a calculated limit value of 0.055 µS/cm, equal to a resistance of 18.2 MΩcm (megohm-centimeters). By comparison, the resistance of distilled water is between 0.5 and 5 µS/cm at 25 °C.

Distilled water is used for less demanding applications, such as filling irons or batteries. The difference in purity level and manufacturing methods makes ultrapure water the preferred choice for high-quality scientific and pharmaceutical purposes, while distilled water is used in less demanding situations.