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HVAC engineers are challenged to design air distribution systems with optimal performance while satisfying a long list of requirements, including maintaining the desired indoor air quality, thermal and acoustical comfort, minimizing energy usage, and life-cycle cost. One major part of the air distribution system is the ductwork system. The pressure drops in the ductwork system are vital to consider while designing it. Within this regard, fittings generate substantial pressure losses in the ductwork system. They frequently dominate the pressure drop in the ductwork system. Therefore, having the appropriate fitting design in the system is important to achieve a superior ventilation system. Due to the considerable impact of fittings on the pressure drop in the ductwork system, many associated studies have been conducted. Some of these studies intend to optimize the shape of the fittings to reduce their generated pressure drops. Other studies are devoted to accurately estimating the pressure drop generated from the fitting, as the current estimations using analytical equations can lead to deviations in estimating the pressure drop. However, efficiently sizing the fittings has not received enough consideration in the current design methods. To be more specific, the existing methods do not adequately account for sizing the fittings at an early design stage, which may result in an inefficient ventilation system. Therefore, a review is conducted in this paper to investigate how the existing design methods consider sizing their fittings and establish why considering them at an early design stage results in better-performing designs. • Fittings sizing is overlooked in the current ductwork design methods. • Considering fittings at an early design stage can reduce the fan energy use. • Considering fittings at an early design stage can achieve balanced ductwork designs. • Decreasing the ductwork sizes does not necessarily increase the fan energy usage. [ABSTRACT FROM AUTHOR]