Flow Rate Calculator
Enter either the pipe diameter and velocity or the volume of fluid passing in a given time to calculate the volumetric flow rate.
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How to Calculate Flow Rate
If you’re installing a new filtration system, water softener, or other pipe-based system in your home or business, you will need to know your home’s flow rate to determine the right system size.
Volumetric flow rate, also referred to as volume flow rate, flow rate, or volume velocity, is defined as the volume of fluid passing a point in a given unit of time.[1] The SI unit of flow rate is cubic meters per second (m³/s).
Meaning that a system may be able to handle a given amount or volume of liquid at one time. Knowing what your plumbing flow rate is will enable you to choose a filter or other system type that will work with your plumbing.
Flow rate can be found by simply measuring the amount of fluid that fills a bucket in a given amount of time or using a formula.
Flow Rate Formula
The flow rate of a fluid can be calculated using the volumetric flow rate equation:
Q = A × v
Thus, the volume flow rate Q is equal to the cross-sectional area of the pipe A times the velocity of the fluid v.[2]
To use the flow rate formula, substitute the cross-sectional area of the pipe and the velocity of the liquid. For a round pipe, the cross-sectional area can be found using the formula for circle area A = πr².
For example, let’s find the flow rate for the fluid passing through a 1″ diameter pipe at a velocity of 3 feet per second.
Start by finding the cross-sectional area of the pipe.
area = π × r²
area = 3.14 × (1 ÷ 2)²
area = 3.14 × 0.25
area = 0.785398 in²
Next, it will be easier to solve later if the velocity and the area are in the same units. Convert the area in square inches to square feet by dividing by 144.
A = 0.785398 in² ÷ 144
A = 0.00545 ft²
Finally, substitute the area and velocity in the flow rate equation and solve.
Q = 0.00545 ft² × 3 ft/sec
Q = 0.01635 cubic ft/sec
Thus, the flow rate is 0.01635 cubic feet per second.
Because most systems work in gallons per minute, or GPM, you will need to convert your cubic feet per second to gallons per minute.
Start by multiplying the cubic feet per second by 7.481, then by 60, which will give you cubic feet per gallon and per minute.
For 0.01635 cubic feet/sec, this would be:
0.01635 × 7.481 × 60 = 7.34 GPM
Alternate Flow Rate Formula
An alternative formula to solve flow rate is:
Q = V / t
The volume flow rate Q is equal to the volume of liquid passed V divided by the time passed t.
To use this formula, you need to be able to measure the volume of liquid that passes in a given time.
For example, let’s find the flow rate for a garden hose that can fill a 5-gallon bucket in 3 minutes.
Q = 5 / 3
Q = 1.67 gal/min
Thus, the flow rate is 1.67 gallons per minute.
Average Flow Rate Chart
While water pressure and other factors can influence your total flow volume, most pipes will have general averages for flow rates. The following assumes minimum and maximum pressures on the pipe:
| Pipe Diameter | Average GPM |
|---|---|
| 1″ | 16 – 58 |
| 1 1/2″ | 35 – 126 |
| 2″ | 55 – 200 |
| 3″ | 140 – 425 |
| 4″ | 240 – 700 |
References
- Engineers Edge, Fluid Volumetric Flow Rate Equation, https://www.engineersedge.com/fluid_flow/volumeetric_flow_rate.htm
- Chris Deziel, How to Calculate Flow Rates, Sciencing, https://sciencing.com/calculate-flow-rates-6199676.html