Pipe Diameter Calculator
Determine the required internal pipe diameter based on volumetric flow rate and fluid velocity.
(Assuming Schedule 40 Pipe)
How to Calculate Pipe Size from Flow Rate
Sizing a pipe correctly is a critical step in process engineering, plumbing, and irrigation. If the pipe is too small, the fluid velocity will be too high, causing excessive pressure drop (head loss), noise, and potential erosion of the pipe material. If the pipe is too large, it results in unnecessary material costs and potential sedimentation if the velocity is too low to sweep solids away.
The Core Formula
The relationship between pipe diameter, flow rate, and velocity is derived from the continuity equation of fluid mechanics:
Q = A × v
Where:
- Q = Volumetric Flow Rate
- A = Cross-Sectional Area of the pipe
- v = Fluid Velocity
Calculating Diameter (Imperial Units)
When working with standard US units—Gallons Per Minute (GPM) for flow and Feet per Second (ft/s) for velocity—the formula to find the internal diameter (d) in inches is:
d = √ [ (0.4085 × Q) / v ]
Here is a practical example:
- Flow Rate: 100 GPM
- Target Velocity: 7 ft/s
- Calculation: (0.4085 × 100) / 7 = 5.835
- Square Root: √5.835 ≈ 2.41 inches
In this scenario, you would need a pipe with an internal diameter of at least 2.41 inches. A 2.5-inch Schedule 40 pipe has an internal diameter of roughly 2.469 inches, which would be a suitable choice.
Recommended Velocity Guidelines
Since velocity is a variable you must choose, it helps to know industry standards to prevent system issues:
- General Water Supply: 4 to 7 ft/s
- Pump Discharge: 8 to 12 ft/s
- Pump Suction: 2 to 4 ft/s (kept low to prevent cavitation)
- Drainage / Gravity Flow: 2 to 3 ft/s
Difference Between NPS and Internal Diameter
It is important to remember that pipes are sold by Nominal Pipe Size (NPS). For NPS 12 and under, the outer diameter is fixed, and the wall thickness varies by "Schedule" (e.g., Sch 40, Sch 80). As the schedule (thickness) increases, the internal diameter decreases.
This calculator provides the required internal diameter. When selecting the physical pipe, ensure the Schedule 40 or 80 pipe you choose has an actual ID greater than or equal to your calculated result to maintain your desired velocity.