Water Flow Rate Calculator
Calculate the volumetric flow rate of water through a pipe based on diameter and flow velocity.
Please enter valid positive numbers for all fields.
Millimeters (mm)
Inches (in)
Meters (m)
Meters per Second (m/s)
Feet per Second (ft/s)
Calculation Results
Cubic Meters per Hour:
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Liters per Minute:
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US Gallons per Minute (GPM):
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Cubic Feet per Second (CFS):
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Pipe Cross-Sectional Area:
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Understanding Water Flow Rate Calculations
Calculating the flow rate of water through a pipe is a fundamental task in fluid mechanics, plumbing, and civil engineering. The flow rate determines how much volume of fluid passes through a cross-section of a pipe over a specific period.
The Continuity Equation
For incompressible fluids like water, the relationship between flow rate (Q), the pipe's internal cross-sectional area (A), and the fluid velocity (v) is defined by the continuity equation:
Q = A × v
Where:
- Q is the volumetric flow rate (e.g., m³/s).
- A is the cross-sectional area of the pipe (e.g., m²).
- v is the average velocity of the fluid (e.g., m/s).
How to Calculate Cross-Sectional Area
Since pipes are circular, the area is calculated using the internal diameter (D). The formula for the area of a circle is:
A = π × (D / 2)² OR A = (π × D²) / 4
Note: It is critical to use the internal diameter of the pipe, not the outer diameter, as the wall thickness reduces the available flow area.
Common Units for Flow Rate
Depending on the application, flow rate is expressed in various units:
- Liters per Minute (L/min): Common for residential plumbing and small pumps.
- Cubic Meters per Hour (m³/h): Standard for industrial processes and municipal water supply.
- Gallons per Minute (GPM): Widely used in the United States for HVAC and irrigation.
- Cubic Feet per Second (CFS): Often used for large open channels or river flows.
Factors Affecting Flow Rate
While the calculation above gives the theoretical flow rate based on velocity and area, real-world systems are influenced by:
- Friction Loss: As water rubs against the pipe walls, pressure is lost, which can reduce velocity if not compensated by a pump.
- Pipe Material: Smoother pipes (like PVC) allow higher velocities with less pressure drop compared to rougher pipes (like concrete or old cast iron).
- Viscosity: While water has a consistent viscosity at standard temperatures, temperature changes can slightly alter flow characteristics.