Typical water meters are mechanical flow meters. Commonly used meters are piston meters, also known as rotary piston or semi-positive displacement meters. The piston meter operates on the principle of a piston rotating within a chamber of known volume. For each rotation, a count for the amount of liquid is computed. Another mechanical meter type is a multijet water meter which operates on the principle of multiple jets of water entering the measuring chamber, where the flow of water causes a rotor to rotate. The rotation of the rotor is then converted into volumetric measurements. Multijet water meters are known for their accuracy and reliability in situations with varying flow rates and they are also relatively resistant to sediment or debris water.
Ultrasonic water meters operate by sending ultrasonic pulses through the water flow and measuring the time it takes for the pulses to travel between sensors located along the pipe. By analyzing the time difference between the upstream and downstream pulses, the meter can accurately determine the velocity of the water flow. Ultrasonic water meters are known for their high accuracy and wide measurement range. A key advantages of ultrasonic water meters is that they do not have any moving parts in contact with the water which minimizes the risk of clogging or degradation over time.

Various prepaid water meter are available depending on the nominal bore (NB). NB (nominal bore) also refers to Nominal Pipe Size (NPS). The Nominal Pipe Size (NPS) is a North American set of standard sizes for pipes used for high or low pressures and temperatures also indicating the required wall thickness.
The European designation equivalent to NPS is DN (nominal diameter) in which pipe sizes are measured in millimeters. Based on the pipe size, the outside diameter (OD) and wall thickness can is obtained by use of the following Reference Tables.
Prepaid Water Meters are typically available with the following features:

• Nominal Bore (mm): Available in: 15,20,25
• Maximum flux (m3/h): Available in: 3, 5 or 7
• Water Temperature: 0-40C
• Working pressure: below 0.1MPa
• Mechanical Counters (m3) upto 9999.9999