Pipe Diameter Calculator
m³/h
L/min
L/s
GPM (US)
ft³/s
m/s
ft/s
Typical water velocity: 1.5 – 3.0 m/s (Suction) | 2.0 – 4.0 m/s (Discharge)
Required Internal Diameter
Cross-Sectional Area:
How to Calculate Pipe Diameter from Flow Rate
Sizing a pipe correctly is a fundamental task in fluid mechanics, plumbing, and process engineering. The relationship between pipe diameter, flow rate, and fluid velocity is governed by the Continuity Equation.
The Core Formula
The basic equation connecting these three variables is:
Q = A × v
Where:
- Q = Flow Rate (volume per time, e.g., m³/s)
- A = Cross-sectional Area of the pipe (e.g., m²)
- v = Velocity of the fluid (e.g., m/s)
Since pipes are circular, the Area (A) is calculated as A = π × (d² / 4), where d is the internal diameter. By substituting this into the first equation and solving for diameter, we get:
d = √[ (4 × Q) / (π × v) ]
Why Velocity Matters
You might wonder, "Why do I need to input velocity?" In most design scenarios, the Flow Rate (Q) is a requirement driven by the process (e.g., you need to move 100 liters of water per minute). However, the diameter isn't fixed until you decide how fast the fluid should move.
- Velocity too high: Results in noise, vibration, pipe erosion, and significant pressure loss (head loss) due to friction.
- Velocity too low: Requires unnecessarily large (and expensive) pipes and may allow suspended solids to settle out of the fluid (sedimentation).
Common Design Velocities:
- General Water Service: 1.0 to 3.0 m/s (3 to 10 ft/s)
- Pump Suction: 0.6 to 1.5 m/s (2 to 5 ft/s)
- Pump Discharge: 1.5 to 3.0 m/s (5 to 10 ft/s)
Calculation Example
Let's say you need to transport water at a rate of 50 m³/h and you want to maintain a velocity of 2 m/s.
- Convert Flow Rate: First, convert 50 m³/h to m³/s.
50 ÷ 3600 = 0.01389 m³/s. - Apply Formula:
d = √[ (4 × 0.01389) / (3.14159 × 2) ]
d = √[ 0.05556 / 6.283 ]
d = √0.00884
d ≈ 0.094 meters - Result: 0.094 meters is 94 mm. In this case, you would select a standard pipe size with an internal diameter slightly larger than 94mm, such as a DN100 (4-inch) pipe.