Pipe Size Calculator
By BRIX Editorial Team · Updated
To size a water line by velocity, add up the gpm of fixtures running at once, then pick the smallest pipe that stays under about 8 ft/s cold or 5 ft/s hot in copper. Two showers, a lavatory and a kitchen sink (9.4 gpm) need a 1 in hot line in Type L copper.
- Design flow
- 9.4 gpm
- 1/2 in (ID 0.545 in)
- 12.9 ft/s too fast
- 3/4 in (ID 0.785 in)
- 6.2 ft/s too fast
- 1 in (ID 1.025 in)
- 3.7 ft/s ok
- 1-1/4 in (ID 1.265 in)
- 2.4 ft/s ok
- 1-1/2 in (ID 1.505 in)
- 1.7 ft/s ok
- 2 in (ID 1.985 in)
- 1 ft/s ok
9.4 gpm through 1 in Type L copper runs at 3.7 ft/s, under the 5 ft/s limit. This is a velocity rule of thumb, not a code sizing: the plumbing code sizes by fixture units, pressure and developed length, and the inspector has the final say.
Key takeaways
- Velocity (ft/s) = 0.4085 × gpm ÷ (inside diameter in inches)². Smaller bore, faster water.
- Copper Development Association: don't exceed 8 ft/s in cold water or 5 ft/s in hot water up to about 140 °F; 2–3 ft/s above that.
- Uponor recommends up to 10 ft/s cold and 8 ft/s hot in its PEX, with a 12 ft/s maximum.
- Nominal size isn't bore: 3/4 in Type L copper is 0.785 in inside; 3/4 in PEX is 0.671 in.
- This is a rule of thumb. The plumbing code sizes by fixture units, available pressure and developed length.
How do you size a water pipe by velocity?
Estimate the flow the pipe must carry when the fixtures on it run together, then find the smallest pipe whose inside diameter keeps that flow under a velocity limit. Too fast and the line is noisy, hammers and (in copper hot lines) can erode; too big and you pay for pipe and wait for hot water.
v = 0.4085 × Q ÷ d²
- v
- water velocity in feet per second
- Q
- flow in gallons per minute
- d
- inside diameter in inches (not the nominal size)
- 0.4085
- converts gpm and square inches to ft/s
- 1
Add up simultaneous flow.
Count the fixtures likely to run at the same time on that line. Federal maximums are 2.5 gpm for a showerhead and 2.2 gpm for lavatory and kitchen faucets; WaterSense fixtures are 2 and 1.5.
- 2
Pick the material and temperature.
Copper and PEX have different bores and different velocity guidance. Hot water gets a lower limit than cold.
- 3
Set the velocity limit.
CDA: 8 ft/s cold, 5 ft/s hot for copper. Uponor: 10 cold, 8 hot recommended for its PEX.
- 4
Check each size.
Compute v for each inside diameter and take the smallest size that stays under the limit.
- 5
Then do it the code way.
Confirm with the fixture-unit method, pressure and developed length. Long runs and low pressure can force a larger size than velocity alone.
How many gpm can each pipe size carry?
| Size | Copper cold (8 ft/s) | Copper hot (5 ft/s) | PEX cold (10 ft/s) | PEX hot (8 ft/s) |
|---|---|---|---|---|
| 1/2 in | 5.8 | 3.6 | 5.5 | 4.4 |
| 3/4 in | 12.1 | 7.5 | 11 | 8.8 |
| 1 in | 20.6 | 12.9 | 18.2 | 14.6 |
| 1-1/4 in | 31.3 | 19.6 | 27.2 | 21.8 |
| 1-1/2 in | 44.4 | 27.7 | 37.9 | 30.3 |
| 2 in | 77.2 | 48.2 | 65 | 52 |
Inside diameters: copper vs. PEX vs. PVC
| Nominal | Type L copper | PEX SDR-9 | Schedule 40 PVC |
|---|---|---|---|
| 1/2 in | 0.545 | 0.475 | 0.622 |
| 3/4 in | 0.785 | 0.671 | 0.824 |
| 1 in | 1.025 | 0.862 | 1.049 |
| 1-1/4 in | 1.265 | 1.054 | 1.38 |
| 1-1/2 in | 1.505 | 1.244 | 1.61 |
| 2 in | 1.985 | 1.629 | 2.067 |
PEX's thick SDR-9 wall gives it a noticeably smaller bore than copper of the same nominal size: 1/2 in PEX has about 76% of the flow area of 1/2 in Type L copper. That is why PEX systems are often home-run from a manifold in 1/2 in lines, one per fixture, rather than trunk-and-branch.
Fixture flow rates
| Fixture | Federal maximum (10 CFR 430.32) | WaterSense |
|---|---|---|
| Showerhead | 2.5 gpm at 80 psi | 2 gpm |
| Lavatory faucet | 2.2 gpm at 60 psi | 1.5 gpm |
| Kitchen faucet | 2.2 gpm at 60 psi | no WaterSense specification yet |
Common pipe sizing mistakes
- Sizing by nominal size. Velocity depends on inside diameter. 3/4 in PEX and 3/4 in copper are different pipes.
- Assuming everything runs at once. Two showers and a sink is realistic; every fixture in the house is not. The code's fixture-unit method handles probability; use judgment here.
- Ignoring pressure and length. A 1/2 in line may pass the velocity check and still starve a second-floor shower on a long run at 40 psi.
- Running hot copper fast. CDA's 5 ft/s hot limit exists because hot water erodes copper faster, and it drops to 2–3 ft/s above 140 °F, as in recirculation loops.
- Forgetting the water heater and meter. The service, meter and heater connections cap flow regardless of branch size.
Frequently asked questions
What size water line do I need for a house?
Most single-family homes are served by a 3/4 in or 1 in service and main, with 1/2 in or 3/4 in branches, but the right answer comes from the plumbing code's fixture-unit method with your available pressure and developed length. Use the velocity check here to sanity-check a branch, not to size a service.
How many gpm can a 3/4 inch pipe handle?
By velocity, 3/4 in Type L copper carries about 12.1 gpm at 8 ft/s (cold) and 7.5 gpm at 5 ft/s (hot). 3/4 in PEX carries about 11 gpm at Uponor's 10 ft/s cold recommendation. Pressure and run length can reduce that.
What is the maximum water velocity in copper pipe?
The Copper Development Association says not to exceed 8 ft/s in cold water and 5 ft/s in hot water up to about 140 °F, and to hold 2–3 ft/s where water routinely runs hotter than that. Excess velocity causes noise and erosion-corrosion.
Is 1/2 inch PEX enough for a shower?
A single showerhead at 2.5 gpm runs about 4.5 ft/s in 1/2 in PEX, within Uponor's recommendations. A dedicated home run from a manifold is the usual approach; a 1/2 in trunk feeding several fixtures at once is not.
Why is PEX smaller inside than copper?
PEX is made to SDR-9, so its wall is one ninth of its outside diameter. 3/4 in PEX has a 0.671 in bore versus 0.785 in for 3/4 in Type L copper, about 27% less flow area, so velocities and friction losses are higher at the same flow.
Does this calculator size gas or drain pipe?
No. It only handles pressurized water supply by velocity. Gas is sized by BTU load, length and the code's gas pipe tables; drain, waste and vent sizing uses drainage fixture units and slope.
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Sources
- Copper Development Association — Copper Tube Handbook (velocity limits; Type L dimensions, Table 14.2b) (checked October 12, 2026)
- Uponor — Plumbing Design Assistance Manual, 4th ed. (SDR-9 PEX dimensions, Table 1-3; velocity guidance) (checked October 12, 2026)
- Westlake Pipe & Fittings — Schedule 40 and 80 PVC pipe data sheet (ASTM D1785 dimensions) (checked October 12, 2026)
- eCFR — 10 CFR 430.32, Energy and water conservation standards (faucet and showerhead flow rates) (checked October 12, 2026)
- EPA WaterSense — Showerheads (checked October 12, 2026)
- EPA WaterSense — Bathroom Faucets (checked October 12, 2026)