Flow Rate Calculator
Calculate water flow rate through a pipe.
Last updated 2026-08-21
What Is a Flow Rate Calculator?
A flow rate calculator converts the size of a pipe and the speed of the water into a flow in gallons per minute. You enter the pipe diameter and the water velocity, and the tool returns how much water moves through each minute. It helps homeowners check whether an existing line can feed a new fixture or appliance, and how many gallons per minute a branch can deliver.
How It Works
Flow (ft³/s) = π × (Diameter ÷ 2 ÷ 12)² × Velocity. GPM = Flow ÷ 0.002228.
Example
3/4 in pipe at 4 ft/s flows about 5.5 GPM.
How to Calculate
- Find the inner diameter of your pipe. A standard 3/4 in supply pipe has an inner diameter of about 0.82 in.
- Estimate the water velocity. Supply lines typically move water at 3 to 5 ft/s, so start at 4 ft/s for a realistic figure.
- Enter the diameter and velocity into the tool. A 3/4 in pipe at 4 ft/s flows about 5.5 GPM.
- Compare the result to the fixture demand. A shower and a faucet together can pull 5 to 8 GPM, so a single 3/4 in line may be near its limit.
- Increase the diameter if you need more flow. Moving to a 1 in pipe at the same velocity roughly doubles the flow, giving you headroom for multiple fixtures.
Flow rates for common pipe sizes at typical velocities
| Nominal pipe size | Flow at 4 ft/s | Flow at 5 ft/s |
|---|---|---|
| 1/2 in | 2.5 GPM | 3.1 GPM |
| 3/4 in | 5.5 GPM | 6.9 GPM |
| 1 in | 9.5 GPM | 11.9 GPM |
| 1 1/2 in | 23 GPM | 29 GPM |
| 2 in | 38 GPM | 48 GPM |
Sources
Frequently Asked Questions
What is the flow in a typical home?
A 3/4 in main at typical velocity moves 5–10 GPM — enough for most fixtures.
Why use 0.002228?
It converts cubic feet per second to gallons per minute.
Does pipe length reduce flow?
Yes, friction over long runs reduces flow; this calculator assumes ideal short-run conditions.
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