Pipe Size and Velocity Chart
Velocity is what decides whether a pipe is noisy, and in copper it decides how long the pipe lasts. Above about five feet per second, hot water strips the protective oxide film off copper faster than it reforms and the wall thins from the inside. Cold water tolerates eight.
Generated from the same data the calculators use — a chart and a calculator on this site cannot disagree.
Velocity in feet per second by flow and nominal size
| Flow | 1/2" | 3/4" | 1" | 1-1/4" | 1-1/2" | 2" | 2-1/2" | 3" |
|---|---|---|---|---|---|---|---|---|
| 2 GPM | 2.7 | 1.3 | 0.8 | 0.5 | 0.4 | 0.2 | 0.1 | 0.1 |
| 4 GPM | 5.5 | 2.6 | 1.6 | 1.0 | 0.7 | 0.4 | 0.3 | 0.2 |
| 6 GPM | 8.2 | 4.0 | 2.3 | 1.5 | 1.1 | 0.6 | 0.4 | 0.3 |
| 8 GPM | 11.0 | 5.3 | 3.1 | 2.0 | 1.4 | 0.8 | 0.5 | 0.4 |
| 10 GPM | 13.7 | 6.6 | 3.9 | 2.5 | 1.8 | 1.0 | 0.7 | 0.5 |
| 15 GPM | 20.6 | 9.9 | 5.8 | 3.8 | 2.7 | 1.6 | 1.0 | 0.7 |
| 20 GPM | 27.5 | 13.2 | 7.8 | 5.1 | 3.6 | 2.1 | 1.3 | 0.9 |
| 30 GPM | 41.2 | 19.9 | 11.7 | 7.6 | 5.4 | 3.1 | 2.0 | 1.4 |
| 40 GPM | 54.9 | 26.5 | 15.5 | 10.2 | 7.2 | 4.1 | 2.7 | 1.9 |
| 60 GPM | 82.4 | 39.7 | 23.3 | 15.3 | 10.8 | 6.2 | 4.0 | 2.8 |
Velocity limits by material and service
| Material | Cold water | Hot water | Limiting factor |
|---|---|---|---|
| Copper (type L) | 8 ft/s | 5 ft/s | Erosion-corrosion is the limit — hot water above 5 ft/s wears the pipe wall from the inside |
| PEX | 12 ft/s | 8 ft/s | Not subject to erosion-corrosion; noise and water hammer set the practical limit |
| CPVC | 8 ft/s | 6 ft/s | Smooth bore, but joints and fittings restrict more than copper |
| Galvanized steel | 8 ft/s | 5 ft/s | Roughens with age — C drops toward 80 in old systems, cutting capacity |
How to read this chart
Nominal size is not inside diameter. These velocities use actual bores — a nominal 3/4 inch type L copper tube has an inside diameter closer to 0.785 inches, and velocity depends on the square of that number.
Erosion-corrosion is the reason copper hot lines are limited more tightly than cold. It fails at elbows first, where turbulence is highest, and shows up as a pinhole leak years after installation.
PEX is not subject to erosion-corrosion, so its practical limit is set by noise and water hammer rather than pipe life. That is why the same flow can be run in a smaller PEX line than copper.
Galvanized steel roughens as it ages. Its Hazen-Williams C factor falls from about 120 when new toward 80, so an old steel system delivers noticeably less flow at the same pressure than these figures suggest.
Sources
- Velocity formula v = 0.408 × GPM ÷ d²
- Accepted erosion-corrosion velocity limits for copper tube
- Hazen-Williams C factors by material
Reference values for planning. Plumbing codes size supply piping by fixture units and available pressure. Not a substitute for a licensed plumber.
Frequently asked questions
What velocity is too high?
For copper, five feet per second in hot water and eight in cold. Above those figures erosion-corrosion becomes the governing concern rather than noise or pressure loss.
Why is hot water more damaging than cold?
Copper protects itself with a thin oxide layer. Hot, fast-moving water removes that layer faster than it can reform, so the bare metal beneath erodes continuously.
Does velocity cause water hammer?
It makes it worse. The pressure spike when a valve closes is proportional to the velocity being stopped, so high-velocity systems bang harder and need arrestors more urgently.
How do I work out my flow rate?
Not by adding every fixture. Plumbing codes convert fixture units to expected demand using a probability curve. For a small house, 8 to 12 GPM is a common design figure for the main.