Pipe Flow Velocity Calculator

Calculate water velocity in a pipe using flow rate and internal diameter.

Calculator

Flow Rate (GPM)

Pipe Internal Diameter (in)

Pipe Velocity

0.0 ft/s

Enter values above and click calculate.

How It Works

This calculator determines how fast water is moving through a pipe given a flow rate and internal pipe diameter. Velocity is a critical factor in plumbing design, too fast causes noise, erosion, and pressure loss, too slow can lead to sediment buildup and poor system performance.

Velocity (ft/s) = Flow Rate (GPM) × 0.4085 ÷ Pipe ID (in)²

Where:
Flow Rate (GPM) = volume of water flowing through the pipe in gallons per minute
0.4085 = unit conversion factor — accounts for pipe cross-sectional area and unit conversions between GPM and ft/s
Pipe ID (in) = internal diameter of the pipe in inches — use the actual ID not the nominal size
Result = water velocity in feet per second

Recommended velocity ranges:
Cold water supply: 2–4 ft/s
Hot water supply: 1.5–3 ft/s
Drain and waste lines: 2–4 ft/s
Above 4 ft/s — risk of noise, erosion, and water hammer

Common example: 10 GPM through a 1" pipe (ID: 1.049 in) results in approximately 3.72 ft/s - within the recommended range for cold water supply.