Water metering is the practice of measuring water use. Water meters measure the volume of water used by residential and commercial building units that are supplied with water by a public water supply system. They are also used to determine flow through a particular portion of the system. In most of the world water meters are calibrated in cubic metres (m3) or litres, but in the United States and some other countries water meters are calibrated in cubic feet (ft.3) or US gallons on a mechanical or electronic register. Modern meters typically can display rate-of-flow in addition to total volume.