Accurate ventilation rate data are essential to maximizing the quality of aerial emission measurements. The fan assessment numeration system (FANS) devices have been widely used by U.S. researchers in measuring aerial emissions from mechanically ventilated livestock and poultry confinement. It is used to conduct in-situ calibration of building ventilation fans and thus development of the fan performance curve under the field operation conditions. The FANS device was originally intended to be placed in the upstream of the fan under in-situ calibration. However, certain field situations make it impractical to apply the FANS device as such. This study assessed the possible use of the FANS in the downstream of a ventilation fan, with the gaps between the FANS device and the discharge cone of the exhaust fan sealed by non-permeable fabric. Nine exhaust fans (1.22 - 1.32 m diameter) in laying-hen and turkey houses were tested with the FANS placed in upstream or downstream for a building static pressure range of 10 to 40 Pa. The results revealed that downstream placement of FANS device yielded 0.44 to 3.1% higher ventilation rate when compared to its upstream placement. This discrepancy is considered acceptable for in-situ fan calibration.