Friction Loss & Flow

Flow Rate Calculator
Pipe Length =
Pipe Diameter =
Cross Sectional Area =
Flow Rate =
Pipe Velocity =
Friction Factor =
Friction Loss =

Friction Loss/Flow Calculations

Several methods are available to determine the friction losses and estimated water flow rates in pipes. In this section one of the most widely used formulas for friction loss, the Darcy-Weisback equation, will be presented so the user can have an understanding of the use of these formulas and the information they can provide.

The Darcy-Weisback formula is:

Friction Loss (hf ) = f L x V2  
d 2g  
hf = Friction Loss (ft) f = Friction Loss Factor
L = Pipe Length (ft) d = Pipe Diameter (ft)
v = Pipe Velocity (ft/sec)
g = 32.2 lb m - ft (Acceleration of Gravity Factor)
lb f - sec2

A simple method to estimate flow rate is the "drum and stopwatch method." Place a drum or container of sufficient size (30 to 50 gallons) at the end of the pipe and, using a stopwatch, estimate the time to fill the container. This will provide a good method to determine the flowrate that can be used in these calculations if a flowmeter is not available.

Sample Calculations:

Determine the friction loss of water pumped through 1,000 ft. of 4-inch diameter pipe at a 300 gpm flow rate. Flowrate can be determined using a flowmeter or the "drum and stop watch method."

The first step is to determine the velocity based on the pipe's cross-sectional area.

Cross-sectional Areas = π x (pipe radius)² Where π = 3.14
Pipe Diameter = 4-inch = .33 ft
Pipe Radius = 2-inch = .16 ft
Cross-sectional Area = 3.14 x (.16)² = .08 ft²

The second step is to determine the pipe velocity.

Pipe Velocity (V) = Flow rate
Cross Sectional Area

Multiply the 300 gpm flow rate by .002 to convert to ft3/sec
300 gpm x .002 = 0.60 ft3 /sec

Calculate Pipe Velocity
Velocity (ft/sec) = 0.60
Velocity (ft/sec) = 7.5 ft/sec

Using a .02 friction factor, calculate the friction loss
Friction Loss (h f ) = f L x V²
d 2g
Friction Loss (ft) = .02 x 1000 ft x (7.5 ft/sec)²
.33 ft 64.4
Velocity (ft/sec) = 52.9 ft


Other Helpful Calculators

To print this calculator or to use it as a standalone file, please download the Excel format below. (Right-click on the link and select "Save Target As" to download it to your computer.)

Download Excel format


L/S = Liters per Second
gpm = Gallons per Minute
HP = Horsepower
KW = Kilowatt
m/s = Meters per Second
= Cubic Meters
= Square Meters
ft² = Square Feet
ft³ = Cubic Feet


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