In fluid dynamics, the pipe flow equation most commonly used is the **Darcy-Weisbach equation** that prescribes the head loss h_{f} to be:

Where ‘L’ is the pipe length,

‘D’ is the internal pipe diameter,

‘V’ is the average fluid velocity, and

‘f’ is the Moody friction factor (Darcy-Weisbach friction factor) is a non-dimensional factor which is a function of several non-dimensional quantities:

where (ρV D/μ) is the Reynolds number Re, and

‘ε’ is the specific surface roughness of the pipe material.

The Moody friction chart (friction factor vs Reynolds number) is the most convenient method of getting the value of moody friction factor ‘f’. For laminar pipe flows (Reynolds number R less than 2000), f = 64/R; because R head loss in laminar flows is independent of wall roughness.

If the duct or pipe is not of circular cross-section area, an equivalent hydraulic diameter D_{eq} as defined earlier is used for calculating head loss or friction factor.

The Swamee-Jain empirical equation may be used to calculate a pipe design diameter directly. The relationship is:

where hydraulic diameter ‘D’ is a function of:

Roughness height ‘ε’ (m, ft),

Volume flow rate ‘Q’ (m^{3}/s, ft^{3}/s),

Length ‘L’ (m, ft),

Velocity ‘v’ (m/s, ft/s),

Gravitational constant ‘g’ is in (m/s^{2}, ft/s^{2}), and

Head loss ‘h_{f}‘, (m, ft).