Flow Rate Calculator — Pipe Velocity to GPM & L/min
Convert pipe inner diameter and mean fluid velocity to volumetric flow rate in GPM, litres per minute, m³/s, and cubic feet per second — plus Reynolds number and flow regime for water.
Diameter unit
Velocity unit
US gallons per minute through this pipe at the given velocity
- 1
Pipe diameter in metres
2 in → SI = 0.0508 - 2
Cross-section area
π × (0.0508 ÷ 2)² = 0.00202683A = π × r² for a circular cross-section. - 3
Flow rate (m³/s)
0.00202683 m² × 0.9144 m/s = 0.00185333 - 4
Flow rate (GPM)
0.00185333 × 264.172 × 60 = 29.376
How does this calculator work?
Flow rate Q = π × (D/2)² × v. Enter pipe inner diameter and mean velocity — any unit — to get GPM, L/min, m³/s, and ft³/s, plus the Reynolds number and laminar/transitional/turbulent regime for water at 20 °C. Real pressure-drop calculations also need the Darcy-Weisbach equation.
Formula
How this is calculated
The volumetric flow rate Q through a completely filled circular pipe is the pipe's cross-sectional area A multiplied by the mean fluid velocity v: Q = A × v, where A = π × (D/2)². The result in m³/s converts to GPM by multiplying by 264.172 × 60, to L/min by multiplying by 1 000 × 60, and to cfs by multiplying by 35.315.
The Reynolds number Re = ρvD/μ characterises the flow regime. Using water-at-20 °C properties (density 998 kg/m³, dynamic viscosity 1.002 × 10⁻³ Pa·s): Re < 2 300 is laminar (smooth, layered flow described by the Hagen-Poiseuille equation), 2 300–4 000 is transitional, and Re > 4 000 is turbulent (chaotic eddies, requiring the Moody chart or Colebrook-White equation for friction losses).
This formula assumes the pipe is completely full and the given velocity is the average cross-sectional velocity. Real systems require pressure-drop analysis via the Darcy-Weisbach equation to confirm the velocity is achievable with the available head. For partial-flow pipes or open channels, use the Manning equation instead.
Frequently asked questions
Residential plumbing is typically designed for 2–4 ft/s (0.6–1.2 m/s). Velocities above 8 ft/s can cause water hammer and erode fittings; below about 1 ft/s sediment may accumulate in cold-water mains.
Flow rate scales with the square of the diameter at constant velocity (area ∝ D²). Doubling the pipe diameter quadruples the cross-section and, at the same velocity, quadruples the flow rate. This is why main distribution pipes are much larger than branch lines.
Reynolds number is a dimensionless ratio of inertial to viscous forces. It determines whether flow is smooth (laminar, Re < 2 300) or chaotic (turbulent, Re > 4 000). Turbulent flow increases friction losses significantly, requiring the Moody chart or Colebrook equation for accurate pressure-drop calculations.
Also known as
TG we-Calculate Editorial Team. (2026). Flow Rate Calculator — Pipe Velocity to GPM & L/min [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/flow-rate-calculator
TG we-Calculate Editorial Team. "Flow Rate Calculator — Pipe Velocity to GPM & L/min." TG we-Calculate. 2026. https://we-calculate.com/calculator/flow-rate-calculator.
TG we-Calculate Editorial Team, "Flow Rate Calculator — Pipe Velocity to GPM & L/min," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/flow-rate-calculator
@misc{wecalculate_flow_rate_calculator, title = {Flow Rate Calculator — Pipe Velocity to GPM & L/min}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/flow-rate-calculator}}, year = {2026}, note = {TG we-Calculate} }
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