Required flow rate may be simply viewed as the amount of firefighting media required to control and ultimately extinguish a fire. This introduces many variables; more precisely two flow rates need to be considered:
- Critical flow-rate (CFR): typically this would be the absolute minimum amount of firefighting media flow needed to fully suppress a fire at any given level of involvement
- Tactical flow-rate (TFR): the target flow for a primary attack hose line or lines
The actual critical flow rate is dynamic; it is directly related to the phase of the fire and this may be unknown. It also has no built-in safety factor. More relevant is the tactical flow rate, which more accurately represents the flow rates required by firefighters to deal with a given fire in a known compartment.
The concept of firefighting flow rate requirements can be based theoretically in matching the flow of firefighting media against known rates of heat release in compartment fires (measured in megawatts or MW).
It can also be empirically based on fire loads, in established floor space, against the flow of firefighting media needed to suppress fires during their growth or decay stages. The latter is generally a defensive application.
It is recognised that flow rate i.e. the amount of firefighting media, extinguishes fire, not pressure.
Relying on pressure alone as the basis to deliver firefighting media does not provide information on the litres per minute being delivered and may be insufficient to prevent fire growth and spread.
The mathematical calculations for the amount of water required to extinguish a given fire are relatively complex. However, as a fire ground rule of thumb for fires from 50 to 600m2, the following calculation could be considered:
   Optimum flow rate (L/min) = fire area (m2) x 5
For example, in a situation with a fire in an open plan flat measuring 90 m2
   Optimum flow = 90m2 x 5 = 450 L/min
This shows that an estimated flow rate of >450 L/min would be required as a minimum to extinguish the fire.
Flow meters
A flow meter is a device that measures the amount of water passing a given point at any time. Vehicles can be equipped with flow meters for each delivery, giving the pump operator information to manage water more effectively. In summary, flow meters provide the knowledge that firefighters require to operate more safely and effectively. Flow meters automatically measure the flow rate when pumping and can also measure the cumulative flow from delivery.