Turbidity measurement is a relative measurement of the clarity of a liquid, usually water. It is an optical characteristic of water and is defined by the amount of light that is scattered when subjected to a light source. The more light is scattered, the greater the turbidity. Many particles can contribute to turbidity including clay, silt, small inorganic and organic matter, algae, dissolved colored organic compounds and other microscopic organisms. It is usually measured in nephelometric turbidity units (NTU) but FTU (formazin turbidity units) and FAU (formazin attenuated units) may also be used in some applications and countries.

In drinking water treatment, the raw water turbidity is measured as well as the organics (and other physical parameters) to define the general water quality entering the works which may drive the coagulation control. Downstream of clarification, turbidity is monitored to ensure that it is being removed at each stage as during disinfection, turbidity must be low (typically <1NTU) to ensure effective disinfection of the final water. A final water turbidity measurement is often between 0.02 and 0.1NTU.

In natural water sources, turbidity may be measured to assess the level of natural variation in a watercourse or to assess the effects of any discharges into it.

As a relative measurement, the ways it can be measured are relatively well defined so as to minimise variation between different instrument designs. These are defined in standards such as ISO7027 or EPA 180.1.