Ductwork system is the lifelines of HVAC systems, responsible for the efficient distribution of air throughout a building. Proper design and optimisation of these systems are essential for achieving energy efficiency, thermal comfort, and reliable performance. In this chapter, we will discuss the principles, best practices, and considerations involved in designing ductwork and air distribution systems. These rules of thumb provide quick estimates for ductwork design and car park ventilation, but final designs should be verified with local building codes, fire safety standards, and computational simulations where applicable.