## CALCULATING FLOW RATES

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 (HR ^{0.54}**

**/ HF**

^{0.54})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)

### Flow-rate:

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

### Volume:

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

### Pressure:

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

### Length:

Meters x 3.2808 = Feet

Feet x 0.3048 = Meters