Pipe Sizing Calculator

Calculate required pipe internal diameter from flow rate and target velocity.

Calculator

Flow Rate (GPM)

Target Velocity (ft/s)

Required Pipe ID

0.00 in

Enter values above and click calculate.

Sch 40

Sch 80

How It Works

This calculator finds the minimum internal pipe diameter needed to carry a given flow rate at a target velocity. Use it during the design or quoting stage to select the correct pipe size before installation — undersized pipe causes excessive velocity, noise, and pressure loss.

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

Where:
Flow Rate (GPM) = volume of water flowing through the pipe in gallons per minute
0.4085 = unit conversion factor relating GPM and pipe diameter to velocity
Target Velocity (ft/s) = your desired flow speed in feet per second
Result = minimum internal diameter required in inches — always select the next standard size up

Recommended target velocities:
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 at a target velocity of 3 ft/s requires a minimum pipe ID of 1.167 in — the next standard size up is 1¼" Schedule 40 with an ID of 1.380 in.