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 Inner Diameter (inches)

Pipe Length (feet)

Calculation Method

Hazen-Williams C Coefficient

Solve For

Head Loss (feet of water)

Hazen-Williams (flow rate from head loss):

Q = 0.4322 × C × D^2.63 × S^0.54

where S = h_f / L (head loss per unit length), D = diameter (inches), C = roughness coefficient

Hazen-Williams (head loss from flow rate):

h_f = 10.67 × L × Q^1.852 / (C^1.852 × D^4.87)

where Q = flow rate (GPM), D = diameter (inches), L = length (feet)

Darcy-Weisbach:

h_f = 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

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.

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