Pipe Flow Calculator
in
ft/s
ft
Flow Rate
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Flow (% of 100 GPM ref)—
Flow Regime—
Flow Rate (GPM)
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Flow Rate (L/min)
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Flow Rate (ft³/s)
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Flow Rate (m³/h)
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Reynolds Number
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Pipe Cross-Section
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Flow Rate Formula
Q = A × v, where A is the pipe cross-sectional area (π × r²) and v is the flow velocity. At 5 ft/s through a 2-inch pipe, the flow rate is about 9.8 GPM. Doubling the pipe diameter quadruples the flow rate.
Reynolds Number
The Reynolds number predicts whether flow is laminar (<2,100), transitional (2,100–4,000), or turbulent (>4,000). Most plumbing and HVAC flows are turbulent. Laminar flow causes less pressure drop but is only achievable at low velocities in small pipes.
Typical Flow Velocities
Cold water supply: 2–4 ft/s | Hot water supply: 1.5–3 ft/s | Drain lines: 2–4 ft/s | Fire protection: 4–8 ft/s. Staying within these ranges minimizes noise, erosion, and water hammer.