Pipe Flow Rate Calculator Formula
Understand the math behind the pipe flow rate calculator. Each variable explained with a worked example.
Formulas Used
Flow Rate
flow_cfs = area_sqft * velocityFlow Rate
flow_gpm = area_sqft * velocity * 448.831Variables
| Variable | Description | Default |
|---|---|---|
diameter | Inside Diameter(in) | 2 |
velocity | Flow Velocity(ft/s) | 5 |
radius_ft | Derived value= diameter / 2 / 12 | calculated |
area_sqft | Derived value= pi * pow(radius_ft, 2) | calculated |
How It Works
Pipe Flow Rate
Q = A x V
Where A is the cross-sectional area of the pipe (pi x r squared) and V is the average fluid velocity. Convert inches to feet before computing the area. Multiply cu ft/s by 448.831 to get gallons per minute.
Worked Example
A 2-inch inside-diameter pipe with water flowing at 5 ft/s.
- 01Radius in feet = 2 / 2 / 12 = 0.0833 ft
- 02Area = pi x 0.0833 squared = 0.02182 sq ft
- 03Q = 0.02182 x 5 = 0.1091 cu ft/s
- 04GPM = 0.1091 x 448.831 = 48.95 GPM
Frequently Asked Questions
What is a typical residential water velocity?
Most plumbing codes recommend keeping velocity between 4 and 8 ft/s for supply lines. Higher velocities cause noise and accelerated pipe wear.
Does this formula work for any fluid?
Yes, the continuity equation Q = A x V applies to any incompressible fluid. The velocity you enter should reflect actual flow conditions.
Learn More
Guide
Plumbing Pipe Sizing Guide: Water Supply and Drain Lines
Learn how to size plumbing pipes for water supply, drainage, and venting. Covers fixture units, pipe materials, code requirements, and pressure considerations.
Ready to run the numbers?
Open Pipe Flow Rate Calculator