In fire protection engineering, the K-factor formula is used to calculate the discharge rate from a nozzle. Spray Nozzles can be fire sprinklers or water mist nozzles, hose reel nozzles, water monitors and deluge fire system nozzles.

K-Factors calculated in Metric units;

Factor-K is defined as following:

K=Q/Sqrt(10*P),

where, Q is the flow rate with its unit as L/min; P is the operating pressure with unit Mpa.

K-Factors have also been calculated and published in English units of PSI and GPM. Within the United States, only English measurements are used.

Care should be exercised not to intermix K-factors from Metric and English units as the resulting factors are not equivalent or interchangeable.

Let's take an high pressure water mist nozzle as example, the operating pressures are from 10Mpa to 14 Mpa and the K factor is 1.00.

So when the operating pressure of the system is 10 Mpa, the flow rate of the water mist nozzle is 10 L/min.

Since K=Q/Sqrt(10*P) =1.0,

Q=K* Sqrt(10*P)=1.0* Sqrt(10*10)=10 L/min;

When the pressure is 14 Mpa, the flow rate is  

Since K=Q/Sqrt(10*P) =1.0,

Q=K* Sqrt(10*P)=1.0* Sqrt(10*14)=11.83 L/min;

Usually for a certain nozzles, K factor is a constant that make it a useful tool to describe relationship between pressures and flow rates.