Pipe Sizing Calculator Formula
Understand the math behind the pipe sizing calculator. Each variable explained with a worked example.
Formulas Used
Minimum Diameter
min_diameter_in = 2 * sqrt(area_needed / pi) * 12Cross-Section Area
area_sqin = area_needed * 144Variables
| Variable | Description | Default |
|---|---|---|
flow_gpm | Required Flow(GPM) | 20 |
max_velocity | Max Velocity(ft/s) | 6 |
flow_cfs | Derived value= flow_gpm / 448.831 | calculated |
area_needed | Derived value= flow_cfs / max_velocity | calculated |
How It Works
Pipe Sizing
Rearrange the continuity equation Q = A x V to solve for the minimum cross-sectional area, then back-calculate the diameter.
D = 2 x sqrt( Q / (V x pi) )
Always round up to the next standard pipe size available from your supplier.
Worked Example
Need 20 GPM at a maximum velocity of 6 ft/s.
- 01Q = 20 / 448.831 = 0.04456 cu ft/s
- 02A = 0.04456 / 6 = 0.007427 sq ft
- 03D = 2 x sqrt(0.007427 / pi) x 12 = 1.17 in
- 04Choose the next standard size: 1-1/4 inch pipe.
Frequently Asked Questions
What standard pipe sizes are available?
Common nominal sizes are 1/2, 3/4, 1, 1-1/4, 1-1/2, 2, 2-1/2, 3, and 4 inches for residential plumbing.
Why does velocity matter?
Excessively high velocity causes water hammer, noise, and erosion. Most codes limit domestic supply to about 8 ft/s.
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 Sizing Calculator