Pipe Flow Rate & Diameter Calculator
Calculate Flow Rate
Calculate Pipe Diameter
mm
inches
meters
m/s
ft/s
Liters/min
m³/hour
GPM (US)
Liters/sec
m/s
ft/s
Calculated Flow Rate:
Liters per minute:
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Cubic meters per hour:
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US Gallons per minute (GPM):
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Liters per second:
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Required Pipe Diameter:
Internal Diameter (mm):
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Internal Diameter (inches):
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Note: This is the exact calculated internal diameter. Select the next standard pipe size up.
How to Calculate Flow Rate and Pipe Diameter
Understanding the relationship between flow rate, fluid velocity, and pipe diameter is fundamental in fluid mechanics and engineering. Whether you are sizing a pump for an irrigation system, designing HVAC piping, or calculating water supply requirements, these three variables are intrinsically linked.
The Fundamental Formula
The continuity equation for incompressible fluids relates volumetric flow rate (Q) to the pipe's cross-sectional area (A) and the fluid velocity (v):
Q = A × v
Where:
- Q = Flow Rate (e.g., m³/s)
- A = Cross-sectional Area of the pipe (m²)
- v = Velocity of the fluid (m/s)
Calculating Area from Diameter
Since pipes are circular, the Area (A) is calculated using the internal diameter (d):
A = (π × d²) / 4
Note: It is crucial to use the Internal Diameter (ID), not the Outer Diameter (OD), as wall thickness does not contribute to flow capacity.
Practical Guidelines for Fluid Velocity
When using the calculator to size a pipe (calculating diameter), selecting an appropriate velocity is key. If velocity is too high, it causes excessive noise, erosion, and high friction loss (pressure drop). If too low, it may allow sediment to settle and requires unnecessarily large pipes.
- General Water Supply: 1.0 to 2.5 m/s (3 to 8 ft/s)
- Suction Lines (Pump Inlet): 0.6 to 1.2 m/s (2 to 4 ft/s)
- Drainage/Gravity Flow: Often lower, around 0.5 to 1.0 m/s
Example Calculation
If you have a pipe with an internal diameter of 50mm (0.05m) and water flowing at 2 m/s:
- Calculate Area: A = (3.14159 × 0.05²) / 4 = 0.001963 m²
- Calculate Flow (Q): Q = 0.001963 m² × 2 m/s = 0.003927 m³/s
- Convert to Liters/min: 0.003927 × 60,000 = 235.6 L/min