Continuity Equation
Q = A x v. Area A = pi x (D/2)^2. For US units, GPM = A(in^2) x v(ft/s) x 0.4085. Reynolds number Re = v x D / kinematic viscosity (1.08e-5 ft^2/s for water at 60°F). Laminar flow: Re < 2300. Turbulent flow: Re > 4000. Transition: 2300-4000.
Frequently Asked Questions
Flow rate (Q) = pipe cross-sectional area (A) x fluid velocity (v). In US units: Q (GPM) = A (sq in) x v (ft/s) x 0.4085. In metric: Q (L/s) = A (m^2) x v (m/s). This is the continuity equation for incompressible flow.
For residential water supply: 4-8 ft/s (1.2-2.4 m/s) is the typical design range. Above 8 ft/s, water hammer and noise become significant concerns. Below 2 ft/s, sediment may settle in horizontal runs. For suction lines to pumps, limit velocity to 4 ft/s to avoid cavitation.
Rearrange Q = A x v: A = Q / v, then solve for diameter: D = 2 x sqrt(A / pi). Choose a standard pipe size that meets or exceeds this diameter. This calculator does the math and recommends the next standard pipe size up.
Nominal pipe size (NPS) is a name, not an exact dimension. A 1-inch NPS pipe does not have a 1-inch inside diameter. The actual ID depends on the pipe schedule (wall thickness). For example, Schedule 40 1-inch pipe has an ID of approximately 1.049 inches. Always use actual ID for flow calculations.
Friction loss (pressure drop) increases with flow velocity, pipe length, and surface roughness, and decreases with pipe diameter. The Hazen-Williams equation is commonly used for water: head loss = 10.67 x L x Q^1.852 / (C^1.852 x D^4.87), where C is the Hazen-Williams coefficient (130-150 for new copper/PVC, 100 for old steel). This calculator covers velocity and flow rate; use the Pipe Pressure Loss Calculator for friction analysis.
Related Calculators
More tools in the Plumbing & Water category.