Pipe Size Calculator (Flow Rate)
Calculate required pipe diameter based on flow rate and velocity.
How to Calculate Pipe Size from Flow Rate
Correctly sizing a pipe is a critical step in designing fluid systems, whether for residential plumbing, industrial process piping, or irrigation. The relationship between pipe size, flow rate, and fluid velocity is governed by the continuity equation.
The fundamental formula used in this calculator is:
Q = A × v
- Q = Volumetric Flow Rate
- A = Cross-Sectional Area of the pipe
- v = Fluid Velocity
Understanding the Variables
To find the required pipe size (Diameter), we rearrange the formula to solve for Area, and then for Diameter:
1. Flow Rate (Q): This is the volume of fluid moving through the pipe per unit of time. In the US, this is commonly measured in Gallons Per Minute (GPM). In metric systems, it is often Cubic Meters per Hour (m³/h).
2. Velocity (v): This is the speed at which the fluid travels inside the pipe. Controlling velocity is essential for system health:
- Too High: Causes noise, pipe erosion, water hammer, and high pressure drop (energy loss).
- Too Low: Allows sediment to settle and requires unnecessarily large (and expensive) pipes.
Recommended Velocities
While specific applications vary, general engineering guidelines suggest:
- Water Suction Lines: 2 to 5 ft/s (0.6 to 1.5 m/s)
- Water Discharge/Pump Lines: 5 to 10 ft/s (1.5 to 3.0 m/s)
- General Water Service: 3 to 8 ft/s (1.0 to 2.5 m/s)
The Calculation Formula
When using Imperial units (GPM and ft/s), the derived formula for the Internal Diameter (in inches) is:
Diameter (inches) = √ [ (0.4085 × Flow Rate GPM) / Velocity ft/s ]
This calculator performs this math instantly to help you determine the minimum required internal diameter (ID) for your specific flow requirements.