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Pipe size for GPM calculator
Smallest standard pipe diameter that carries your target flow rate (gpm) without exceeding the velocity ceiling (default 8 ft/s for copper, 10 ft/s for steel). Covers common copper Type L and steel schedule 40 sizes.
Recommended pipe size
1-1/2" (Copper Type L)
At 40 gpm, ID = 1.505″, velocity = 7.21 ft/s (ceiling 8 ft/s).
Velocity = gpm ÷ (cross-section-sq-ft × 448.83). Ceilings are rules of thumb (erosion + noise). For a fully designed system, the plumbing engineer runs friction-loss + diversity per the connected fixture-unit count.
Velocity ceilings used
- Copper hot water: 5 ft/s (erosion guard)
- Copper cold water: 8 ft/s
- Steel water: 10 ft/s
- PVC drain/waste: 6 ft/s
Formula: velocity (ft/s) = gpm / (cross-sectional-area in sq-ft × 448.83). The tool inverts the formula and picks the smallest size that stays under the velocity ceiling.
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Plumbing math + project lookup over text.
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