Water Velocity Calculator
Enter the flow rate and diameter of the pipe to calculate the velocity of the water traveling through.
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How to Calculate Water Velocity
Water velocity is a measure of the rate of water flowing through a closed pipe system.[1] Knowing the water velocity of a pipe can help determine things like pressure. Having the right velocity is important if you want to extend the life of your pipes; water velocity that is too high for your system can result in the pipes corroding faster and needing to be replaced sooner.
Water Velocity Formula
Water velocity for a pipe of any size or length can be found using a simple formula:
v = Q / A
Thus, the velocity v is equal to the flow rate Q divided by the cross-sectional area A.
When using this formula, all units should be in the same form of measurement. If the flow rate is measured in cubic meters per second, then the area should be in square meters, and if the flow rate is measured in cubic feet per second, then the area should be in square feet.
The cross-sectional area of a pipe can be found using the formula:
A = πr²
The area A is equal to π times the radius r squared.
For example, let’s find the water velocity given a flow rate of 35 cubic feet per second and a 1″ diameter pipe.
Start by finding the cross-sectional area of the pipe. You will need to convert the diameter from inches to feet, then divide this in half to find the radius.
diameter in feet = 1″ ÷ 12 = 0.0833′
radius = 0.0833′ ÷ 2 = 0.04167′
area = π × 0.04167² = 0.0054542 sq ft
Next, calculate the velocity.
v = 35 / 0.0054542
v = 6417.1 ft/s
Alternative Formula
The formula above works great when the flow rate is measured in the cubic form of a standard length unit. But when the flow rate is measured in gallons per minute or liters per minute, a different method is needed.
In this case, start by converting the flow rate to cubic feet per second or cubic meters per second.
Alternatively, if the flow rate is measured in gallons per minute, the following formula can be used to calculate water velocity.
v = 0.408 × Q / D²
Thus, the velocity v in feet per second is equal to 0.408 times the flow rate Q in gallons per minute (GPM) divided by the pipe diameter D in inches squared.[2]
For example, let’s find the water velocity given a flow rate of 15 GPM and a 0.75″ diameter pipe.
Start by finding the cross-sectional area of the pipe.
v = 0.408 × 15 / 0.75²
v = 0.408 × 15 / 0.5625
v = 6.12 / 0.5625
v = 10.88 ft/s
So, the velocity of water inside a 0.75″ pipe with a flow rate of 15 GPM is equal to 10.88 ft/s.
References
- Hunter, The Handbook of Technical Irrigation Information, https://www.hunterindustries.com/sites/default/files/tech_handbook_of_technical_irrigation_information.pdf
- Ace Pumps, Useful Calculations and Conversions, https://www.acepumps.com/en/index.php?/site/applications/C34/Calc