Pipe Flow Rate Calculator
Calculate fluid velocity and volumetric flow rate through circular pipes.
Cubic Feet per Second (CFS):
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Gallons per Minute (GPM):
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Liters per Minute (LPM):
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Cubic Meters per Hour (m³/h):
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How to Calculate Flow Rate in a Pipe
Calculating the flow rate of water or other fluids in a pipe is a fundamental task for plumbers, irrigation specialists, and engineers. The flow rate describes the volume of fluid that passes through a specific cross-section of the pipe over a set period of time.
The Flow Rate Formula
The standard formula for volumetric flow rate is:
Q = A × v
- Q: Volumetric flow rate
- A: Cross-sectional area of the pipe
- v: Velocity of the fluid
For a circular pipe, the Area (A) is calculated using the diameter (D):
A = π × (D/2)²
Step-by-Step Calculation Example
Suppose you have a 4-inch (internal diameter) pipe with water moving at a velocity of 6 feet per second (ft/s). Here is how you calculate the flow rate:
- Calculate Radius: Diameter is 4 inches, so radius is 2 inches.
- Convert to Feet: 2 inches / 12 = 0.1667 feet.
- Calculate Area: A = π × (0.1667)² = 0.0873 square feet.
- Calculate Flow (CFS): Q = 0.0873 sq ft × 6 ft/s = 0.5238 Cubic Feet per Second.
- Convert to GPM: 0.5238 CFS × 448.83 = 235.1 Gallons per Minute.
Standard Pipe Flow Velocity Guidelines
In most pressurized systems, velocity is kept within specific ranges to prevent pipe erosion or "water hammer" (noise and vibration).
| Application | Recommended Velocity (ft/s) |
|---|---|
| Residential Water Supply | 4 – 8 ft/s |
| Irrigation Mainlines | 5 – 7 ft/s |
| Suction Lines (Pump) | 2 – 4 ft/s |
| Drainage (Gravity Flow) | 2 – 5 ft/s |
Factors Affecting Flow Rate
While the formula above is mathematically perfect, real-world flow rates are affected by several variables:
- Pipe Friction: The roughness of the pipe material (PVC vs. Cast Iron) creates drag.
- Viscosity: Thicker fluids (like oil) flow differently than water.
- Fittings and Valves: Elbows and tees create turbulence that slows down velocity.
- Pressure Differential: The difference in pressure between the start and end of the pipe drives the velocity.