Flow Rate Calculator
Need to figure out how much fluid your system is moving? Use our flow rate calculator to get accurate numbers fast and take the guesswork out of HVAC and plumbing jobs.
Just plug in your pipe size and flow velocity, and the calculator will estimate the flow rate for you.
Disclaimer:
The outputs, any associated images or diagrams, any measurements, and any and all other information (collectively, the “Outputs”) are provided for informational purposes only and do not constitute a binding quote or professional advice. The Outputs are based on general assumptions and the information you provided, which may not reflect the actual specifications or condition of the subject of the calculation, particularly in the case of complex structures or inputs. Accuracy may also vary depending on location, as some data may be more complete in certain regions than others. Actual results or outcomes may vary significantly following an onsite inspection and detailed assessment by a qualified professional. Any auto-generated visuals are for illustrative purposes and may not accurately represent the subject of the calculation. The Outputs should not be relied upon as a final result or used as the sole basis for financial, renovation, operational, or strategic decisions. Jobber recommends consultation with a qualified professional before proceeding with any work, quotes, or financial decisions. Jobber and its affiliates make no warranties, express or implied, regarding the accuracy, completeness, or suitability of the Outputs, and to the fullest extent permitted by law, we disclaim all liability for any damages or losses arising from reliance on the Outputs.
How to use this flow rate calculator
Quoting a job and need to know if that 4-inch PVC line will handle the system’s water demand? This calculator helps you quickly estimate flow rate without the guesswork.
Whether you’re working on plumbing systems or fluid-handling components in HVAC, this tool makes it simple to size components and check performance. Here’s how to use the calculator without overthinking it:
- Pick your measurement system (Imperial or Metric): Choose the unit system that matches your jobsite or spec sheet. This step sets the tone for every number you plug in after, so your results stay aligned with the rest of your design.
- Select your pipe dimensions (inside and outside diameter): Flow depends on the inside diameter of a pipe–it’s the space the fluid’s moving through. But, knowing the outside diameter is also important for selecting fittings, insulation, and identifying pipe types in the field. Pick from the dropdowns of common pipe sizes.
- Enter your flow velocity: Here’s where the magic starts. Just type in the velocity—how fast the air or liquid is moving through the pipe. Whether you’re measuring in feet per second or meters per second, the calculator’s flexible. If you don’t have a velocity reading handy, you can get a rough estimate from a flowmeter, a manometer, or even a manufacturer’s spec sheet.
- Hit “Calculate” and get instant answers: Get the volume flow rate in cubic feet or meters, and the liquid flow rate broken down into gallons (or liters) per second, minute, and hour. This calculator is perfect for double-checking a system design or giving your client an answer without disappearing for 20 minutes to do some math.
How to calculate flow rate
You can use the tool above to calculate flow rate, or follow these steps to do it yourself:
- Measure the inside diameter of the pipe: Use a tape measure or calipers to find the pipe’s inside diameter, which will be in inches (Imperial) or millimeters (Metric). Measuring accurately is important, because a small error here can result in a larger one in the final flow rate.
- Calculate the cross-sectional area (A):
A = [ π * (diameter)2 ] / 4
Be sure your measurement units match (inches or millimeters). For metric, convert millimeters to meters by dividing by 1,000.
- Measure or estimate the flow velocity (v):
Flow velocity is typically given in feet per second (ft/s) or meters per second (m/s).
You can use velocity probes, anemometers, or consult system documentation to get this value.
- Bring it all together to calculate the volumetric flow rate (Q):
Q = A * v
Where Q is in cubic feet per second (ft³/s) or cubic meters per second (m³/s).
To convert to gallons or liters, use these conversions:
- 1 cubic foot = 7.4805 gallons
- 1 cubic meter = 1,000 liters
Formula for flow rate calculation
You don’t need a Ph.D. in fluid dynamics to figure out flow rate. You just need the right formula and a few basic inputs. Here’s how the math works:
Flow rate = (cross-sectional area of the pipe) * (velocity of the fluid)
Flow rate = Q = A * v = ( [ π * (diameter)2 ] / 4 ) * (velocity of the fluid in ft/s or m/s)
Or, when you’re already juggling scheduling, inventory, and phone calls, skip the manual calculations and use our calculator above.
Frequently Asked Questions
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Flow rate tells you how much fluid or air moves through a pipe or duct in a given amount of time. It shows your system’s capacity. You’ll usually see it measured in units like GPM, L/s, ft³/s, or m³/h. Calculating flow rate is critical when you’re designing systems or troubleshooting performance issues. A flow rate that’s too high can increase noise or pressure loss—too low can lead to poor performance or overheating in systems like HVAC or plumbing.
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Volumetric flow rate is the volume of fluid moving through a system over time. It’s calculated by multiplying the pipe’s cross-sectional area by the fluid velocity. Knowing the volumetric flow rate keeps your design within spec and prevents issues like pressure loss, uneven heating/cooling, or insufficient water supply.
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Yes. A bigger diameter means more space, which means more flow. At a steady velocity, the flow rate increases with the square of the inside diameter. That’s why selecting the right pipe size can mean the difference between a quiet, efficient system and one that’s noisy or underperforming.
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You want water flow that’s strong but not destructive. For most residential systems, this means keeping the water flow rate below eight feet per second—and often closer to 4-6 feet per second—to avoid things like erosion or hammering. Check manufacturer specs and local plumbing codes to hit the sweet spot between performance and longevity.
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Pipe size and flow rate go hand in hand. At a fixed velocity, increasing the pipe’s inside diameter increases flow rate. That’s why pipes should be sized based on peak demand, not average use. And since pipe specs often list outside diameter, Jobber’s flow rate calculator handles the math to get the accurate inside diameter for precise flow calculations.
What factors affect the flow rate in a pipe?
Several things come into play:
• Pipe diameter and roughness: Larger diameters increase flow capacity, and smoother pipes reduce friction loss.
• Velocity and fluid properties: Faster flow means more volume moves through the pipe. Heavier fluids (higher density) affect system performance but don’t directly change how much volume flows.
• Pressure and pipe layout: More pressure drives flow, but elbows, fittings,and elevation changes create resistance that affects the final output