Pipe Size Calculator
Determine the correct pipe size for flow requirements.
Last updated 2026-08-21
What Is a Pipe Size Calculator?
A pipe size calculator tells you the smallest pipe diameter that can carry your required flow without moving water too fast. You enter the flow rate in gallons per minute and the target water velocity, and the tool returns the pipe size you need. It is a quick planning check for homeowners running a new supply line, branch run, or irrigation feed before buying fittings.
How It Works
Flow (ft³/s) = GPM × 0.002228. Area = Flow ÷ Velocity. Diameter = 2 × sqrt(Area × 144 ÷ π).
Example
12 GPM at 5 ft/s needs ≈1 in pipe.
How to Calculate
- Estimate the flow you need in gallons per minute. Add up the fixtures the line will serve. A bathroom group runs roughly 6 to 8 GPM, a laundry set 4 to 6 GPM, and a kitchen sink 2 to 3 GPM.
- Pick a target velocity. Start with 5 feet per second, the common design target for supply piping. Keep it near 5 for water service lines to avoid noise and erosion.
- Enter the flow rate and velocity into the tool. For a 12 GPM demand at 5 ft/s it returns about a 1 in pipe.
- Check the nearest standard pipe size. If the result falls between nominal sizes, round up to the next standard pipe, for example from 3/4 in to 1 in, to keep velocity comfortable.
- Confirm the fittings and valves on the run are the same size. Any reduction at a valve or elbow becomes the bottleneck and raises velocity at that point.
Recommended water velocities by application
| Application | Typical velocity | Notes |
|---|---|---|
| Water service line | 5 ft/s | Keeps noise and erosion in check |
| Hot water circulation | 2 to 4 ft/s | Slower to limit heat loss and noise |
| Main supply branch | 5 ft/s | Common design target |
| Suction piping | 2 to 3 ft/s | Lower velocity protects pumps |
| Drainage and vent | 0 to 2 ft/s | Gravity flow, not pressurized |
Sources
Frequently Asked Questions
What is a good water velocity?
5 ft/s is a common design target; higher speeds cause noise and erosion.
Why convert GPM to ft³/s?
The area/velocity formula works in consistent units, so gallons per minute is converted to cubic feet per second.
Does material affect sizing?
Not in this simplified calculator — pipe roughness matters for long runs, but size here is driven by flow and velocity.
Estimate the cost with the Plumbing Cost Calculator. Also try: Pipe Volume Calculator, Flow Rate Calculator or browse Plumbing Calculators