Nominal flow rate measures the volume of liquid passing through a system under specific pressure conditions. Depending on region, the nominal flow rate may be measured in gallons per minute, or GPM, or liters per minute, or LPM. High-volume applications may even be expressed in gallons or liters per second. Nominal flow rate calculations can be critical to successful operation, maintenance and design of many types of plumbing or industrial systems.


Manufacturers of pipes, valves, pumps and other plumbing equipment test and certify products based on the nominal flow rate they are designed to provide. This nominal flow rate is calculated by measuring the rate of flow at several different levels of water pressure, then finding the average flow rate. For example, companies may measure rate of flow through a plumbing valve at 150, 250 and 350 kilo pascals, or kPa of pressure. By adding these three flow rates and dividing this figure by three, the manufacturer can provide data on the nominal flow rate for the valve. This helps buyers know whether this valve is appropriate for a specific application.


The nominal flow rate of a plumbing fixture is influenced by a variety of factors beyond water pressure levels. Everything from the size of a pipe or valve to the material is it made from can effect friction levels, which impacts the rate of flow. Other considerations include the velocity of the fluid, the type of fluid and the percentage of solid it contains and the slope or distance it must travel.


Major industrial and plumbing systems use a device called a flow meter to measure nominal flow rate through the system. These devices are built into the system to provide constant information or flow, or to alert maintenance personnel of a problem.

For smaller commercial or residential applications, technicians rely on a portable flow meter to measure rate of flow. These hand-held devices are similar in size to a notebook computer, and serve as a non-invasive method of determining nominal flow rate. The flow meter attaches to pipes or tubes using special clamps or magnets, and sends signals into the pipe using ultrasound waves. The waves meet resistance from the fluid inside and send back data on the rate of flow.


Nominal flow rate information helps users choose the correct plumbing components for each type of system. For example, lawn sprinkler pumps and heads are each rated to a certain rate of flow. These flow rates must match in order for the sprinklers to function as intended. The same is true of sump pumps, faucets, pipes and other fixtures.


While nominal flow rate is commonly associated with fluids, it is also used to refer to the flow of gaseous materials. Nominal flow rate may also be used to describe the efficiency of a vacuum system, where the flow of air is rated in cubic feet or cubic meters per second rather than the GPM figure used to describe the flow of liquids.