Pipe Size & Flow Rate Calculator
Calculate volumetric flow rate, flow velocity, and pressure drop for circular pipes using the Hazen-Williams and Darcy-Weisbach equations.
Pipe Geometry
Flow Parameters
Formulas Used
Hazen-Williams (flow rate from head loss):
Q = 0.4322 × C × D2.63 × S0.54
where S = hf / L (head loss per unit length), D = diameter (inches), C = roughness coefficient
Hazen-Williams (head loss from flow rate):
hf = 10.67 × L × Q1.852 / (C1.852 × D4.87)
where Q = flow rate (GPM), D = diameter (inches), L = length (feet)
Darcy-Weisbach:
hf = f × (L / D) × (V² / 2g)
Friction factor f via Colebrook-White equation (iterative):
1/√f = −2 log₁₀(ε/(3.7D) + 2.51/(Re√f))
Flow Velocity:
V = Q / A = Q / (π × (D/2)²)
Reynolds Number:
Re = V × D / ν
Assumptions & References
- Hazen-Williams is valid for water at typical temperatures (40–75°F) and turbulent flow only.
- Darcy-Weisbach applies to any Newtonian fluid; friction factor solved via Colebrook-White iteration (converges in ~50 iterations).
- Calculations assume full-pipe, steady-state, incompressible flow in a straight horizontal pipe.
- Minor losses (fittings, bends, valves) are not included.
- 1 GPM = 0.002228 ft³/s; 1 inch = 1/12 ft; kinematic viscosity of water ≈ 1.004 cSt at 68°F.
- References: Moody (1944), Colebrook & White (1937), AWWA Manual M11, Crane TP-410.