If you are using a software program to calculate flow, you may need to input a coefficient or k-factor into the program to calculate flow-rate. Using the coefficients will give relatively accurate results. Check results against our flow charts to verify calculations were done correctly. Our flow charts are calculated using K-factors derived from testing at FM Approvals.

If you need the Coefficients, please visit our Hydraulics & Engineering Information page in the Resources section.

Here are the equations used for calculating flow-rates and predicting flow-rates.

K-factor Formula

Computes a flow-rate in GPM given a psi and a K-factor of the flow device.
Q = √P x K
Q = flow-rate in GPM
P = velocity pressure in psi
K = K-factor of flow device

Theoretical Discharge through Circular Orifices Formula

Computes a flow-rate in GPM given a psi and coefficient of the flow device.
Q = 29.84 x √P x D² x C
Q = flow-rate in GPM
P = velocity pressure in psi
D = orifice diameter in inches
C = coefficient of flow device

Equation for Determining Rated Capacity

Computes the flow-rate available at a specified residual
pressure (a.k.a. Rated Capacity). The example below enables
you to find the predicted flow-rate at 20 psi residual pressure.
Use this when fire flow testing hydrants.
QR = QF x (HR0.54 / HF0.54)
QR = Flow-rate predicted at the desired residual pressure in GPM
QF = Total test flow-rate measured during test in GPM (GPM measured from Hose Monster or Pitotless Nozzle)
HR = Pressure drop from static pressure to desired residual pressure (Static – 20 psi [if 20 psi is the desired residual pressure])
HF = Actual pressure drop measured during the test (Static – Actual Residual)


US Gallons per Minute x 3.785 = Liters per Minute
Liters per Minute x 0.264 = US Gallons per Minute
US Gallons per Minute x 0.1337 = Cubic Feet per Minute
Cubic Feet per Minute x 7.481 = US Gallons per Minute


US Gallons x 3.785 = Liters
Liters x 0.264 = US Gallons
US Gallons x 0.8327 = Imperial Gallons
Imperial Gallons x 1.201 = US Gallons
Cubic Feet x 7.48051945 = US Gallons
US Gallons x 0.1337 = Cubic Feet


psi x 0.0689 = Bars
Bars x 14.5038 = psi
psi x 6894.757 = Pascals
Pascals x 0.000145 = psi
Bars x 100,000 = Pascals
Pascals x 0.00001 = Bars

Weight of Water:

US Gallons of Water x 8.3454 = Pounds
Cubic Feet of Water x 62.42796 = Pounds


Meters x 3.2808 = Feet
Feet x 0.3048 = Meters