Flow Rate Calculator

To calculate the flow rate of water inside a closed pipe, contractors such as plumbers, pipefitters, and irrigation specialists must understand complicated math formulas and know how to apply them on the job. ServiceTitan’s water flow rate calculator makes determining volumetric flow rate quick and easy.

The basic flow rate equation: A x v = Q. 

• A = cross-sectional area at a point in the flow’s path

• v = velocity of the liquid at that point

• Q = flow rate

What is Flow Rate?

To understand flow rate in simple terms, imagine measuring the amount of water flowing from a spigot into a 5-gallon bucket over a period of several minutes. Divide 5 by the number of minutes it took to fill the bucket, and you’ll know the flow rate of the water in gallons per minute.

The volume flow rate, also referred to as volumetric flow rate, measures the volume of a fluid that passes through a given cross-sectional area of a pipe over a specific unit of time. Pipe flow rate is usually represented by the letter Q in the flow rate equation.

While a liquid flow rate calculator measures the amount of fluid output in terms of volume, velocity in pipe refers to how fast the fluid is moving in distance per a specific unit of time. 

How to Calculate Flow Rate

To calculate flow rate, you must first determine the area (A) through which the fluid flows and its velocity (v) or speed.

The area represents the cross-sectional space at a point in the flow’s path, or basically the inner diameter of the pipe. 

• Calculate the inner diameter of the pipe by measuring the distance from one inside edge, across the center, and to the opposite inside edge. Measure in inches or millimeters.

The velocity represents how fast the fluid moves from Point A to Point B inside the pipe within a specific period of time. Click here to learn more about how to calculate velocity, or reference this pipe velocity calculator tool.

To clarify the difference between flow rate and velocity in pipe, consider a river’s flow rate. The greater the velocity of the water, the greater the flow rate of the river. But flow rate also depends on the size of the river. A bubbling creek carries far less water than the Mississippi River, for instance.

Flow Rate Formula and Example

Plumbers and other service contractors seek accurate measurements for water flow calculation when working on pipes for plumbing, HVAC, irrigation, and more.

Volume flow rate formula: A (area) x v (velocity) = Q (flow rate)

Here’s a specific example of how to apply the volumetric flow rate equation:

For an 8-inch pipe (diameter) with water flowing through it at an average velocity of 16 feet per second, we can determine flow rate by using the formula: 

Area = pi x (diameter)²/ 4

• pi = 3.1415926 

• Diameter squared = 8 inches out of 12 inches reduces to ⅔ or 0.6666667

  • *You convert inches to feet by dividing 8 by 12, since velocity was measured in feet per second.

• 0.6666667 squared = 0.444444488

• 0.444444488 multiplied by 3.1415926 (pi), then divided by 4 = 0.349 ft²

Now we know A = 0.349 ft²

• Q = Area (A) x velocity (v)

• Q = 0.349 ft² X 16 ft per second

• Q = 5.584 ft³ 

Result: Water flows through the 8-inch pipe at a rate of 5.584 ft³ per second.

Bonus Tip: Measuring Pipe Diameter

If you’re unsure how to measure the inner diameter of a pipe, consider investing in a set of calipers that fit around the outside of the pipe. Use the calipers to measure the outside diameter directly instead of estimating inner diameter based on circumference.

Once you determine the outer diameter, refer to this common pipe dimensions table to clearly find the inner diameter of your pipe.

Flow Rate Equation Simplified

Determining a flow rate calculation simply means determining how fast the water moves through a pipe and how much space it occupies inside the pipe as it moves from one point to another. Use ServiceTitan’s Flow Rate Calculator to save time, reduce waste, and get an accurate measurement every time.

Disclaimer Statement

*The recommended values are in good faith and are solely meant for generic, informative purposes. We do not guarantee the accuracy of this information. Please note that other external factors may affect or falsify the recommendations. For accurate values, consult a licensed engineer.