Recommended Size
1"
Velocity
4.1 ft/s
Friction Loss
1.48 PSI
Remaining
58.5 PSI
1"
4.1 ft/s
1.48 PSI
58.5 PSI
Inputs
Result
Min area = 10 / (5 × 448.831) = 0.00446 ft² = 0.642 in². Min diameter = 0.904". Next standard size: 1". But 3/4" pipe at 10 GPM gives velocity of 7.3 ft/s which exceeds 5 ft/s, so 1" is recommended.
Inputs
Result
Min area = 25 / (5 × 448.831) = 0.01114 ft² = 1.604 in². Min diameter = 1.43". Next standard size: 1-1/2". Velocity at 1.5" = 4.5 ft/s (within limit).
Pipe size depends on flow rate (GPM), maximum velocity (typically 5 ft/s for residential), and acceptable friction loss. Calculate the minimum pipe area from flow rate divided by max velocity, then select the next standard size up.
| Pipe Size | Max Flow (5 ft/s) | Common Use |
|---|---|---|
| 1/2" | 4 GPM | Individual fixtures (faucet, toilet) |
| 3/4" | 10 GPM | Branch lines, small homes |
| 1" | 18 GPM | Main supply, medium homes |
| 1-1/4" | 28 GPM | Larger homes, multi-bath |
| 1-1/2" | 40 GPM | Irrigation, commercial |
| 2" | 72 GPM | Large commercial, fire lines |
The C value represents pipe roughness in the Hazen-Williams friction equation. Smoother pipes have higher C values. PEX and PVC: C=150, copper: C=140, galvanized steel: C=120. Lower C means more friction loss.
| Pipe Material | C Value | Friction Loss Relative |
|---|---|---|
| PEX / PVC | 150 | Lowest (baseline) |
| Copper (new) | 140 | ~15% more than PEX |
| Ductile Iron | 130 | ~30% more than PEX |
| Galvanized (new) | 120 | ~50% more than PEX |
| Galvanized (20+ yrs) | 80–100 | 2–3× more than PEX |
Residential plumbing should keep velocity under 5 ft/s to prevent noise and erosion. Commercial systems allow up to 8 ft/s. Higher velocities cause water hammer, pipe noise, and accelerated corrosion.
Longer pipe runs have more friction loss. A 200 ft run has twice the friction loss of a 100 ft run at the same flow rate. Long runs may require upsizing the pipe to maintain adequate pressure at the fixture.
| Pipe Run Length | Friction Loss (3/4" Cu, 10 GPM) | Action |
|---|---|---|
| 50 ft | 1.9 PSI | Standard 3/4" is fine |
| 100 ft | 3.8 PSI | Standard 3/4" usually OK |
| 200 ft | 7.6 PSI | Consider upsizing to 1" |
| 300 ft+ | 11.4+ PSI | Upsize to 1" or add booster |
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Last Updated: Mar 9, 2026
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