The Darcy-Weisbach equation is a fundamental equation in fluid mechanics used to calculate the pressure loss (head loss) due to friction in a pipe or duct. It relates the head loss to the flow rate, pipe characteristics, and fluid properties. The equation is widely used in engineering, particularly in the design and analysis of fluid flow systems. The general form of the Darcy-Weisbach equation is,

Where:

• hf​ is the head loss due to friction,
• $f$ is the Darcy-Weisbach friction factor, which depends on the flow conditions and the roughness of the pipe surface,
• $L$ is the length of the pipe,
• $D$ is the diameter of the pipe,
• $V$ is the flow velocity, and 
• $g$ is the acceleration due to gravity.