A water meter is a device used to measure the volume of water usage. In many developed countries water meters are used to measure the volume of water used by residential and commercial building that are supplied with water by a public water supply system. Water meters can also be used at the water source, well, or throughout a water system to determine flow through that portion of the system. In most of the world water meters measure flow in Cubic metres (m3) or litres but in the USA and some other countries water meters are calibrated in cubic feet (ft. 3), or US gallons on a mechanical or electronic register. Some electronic meter registers can display rate-of-flow in addition to total usage. There are several types of water meter in common use. Selection is based on different flow measurement methods, the type of end user, the required flow rates, and accuracy requirements