Pipe Flow Rate Calculator
Imperial (Inches / Feet per second)
Metric (Millimeters / Meters per second)
Calculation Results:
Volumetric Flow Rate:
Alternative Unit:
Pipe Cross-Section Area:
How to Calculate Flow Rate Through a Pipe
Understanding the volume of fluid passing through a pipe is critical for plumbing, irrigation, HVAC, and industrial processing. The calculation relies on the relationship between the cross-sectional area of the pipe and the speed (velocity) at which the fluid is moving.
The Pipe Flow Formula
The standard formula used for calculating volumetric flow rate (Q) is:
Q = A × v
- Q: Volumetric Flow Rate
- A: Cross-sectional area of the pipe ($\pi \times r^2$)
- v: Flow Velocity
Step-by-Step Calculation Guide
- Determine Internal Diameter: Measure the inside diameter of the pipe. Note that "nominal" pipe sizes (like Schedule 40) often have actual internal diameters that differ slightly from their names.
- Calculate Cross-Sectional Area: Convert the diameter to radius (r = d/2). Calculate area using $\pi \times r^2$. Ensure your units are consistent (e.g., if velocity is in feet per second, area should be in square feet).
- Measure Velocity: Determine how fast the fluid is moving. In most pressurized systems, water velocity is kept between 2 and 8 feet per second to prevent noise and pipe erosion.
- Multiply: Multiply the area by the velocity to get the flow rate.
Real-World Example
Suppose you have a 2-inch (ID) pipe with water moving at a velocity of 5 feet per second.
- Radius: 1 inch = 0.0833 feet
- Area: $\pi \times (0.0833)^2 = 0.0218$ square feet
- Flow Rate: $0.0218 \times 5 = 0.109$ Cubic Feet per Second (CFS)
- Conversion: 0.109 CFS is approximately 48.9 Gallons per Minute (GPM).
Common Conversion Factors
| From | To | Multiply By |
|---|---|---|
| Cubic Feet per Sec (CFS) | Gallons per Min (GPM) | 448.83 |
| Cubic Meters per Sec (m³/s) | Liters per Minute (LPM) | 60,000 |
| Gallons per Min (GPM) | Liters per Minute (LPM) | 3.785 |