Due to high installation costs and space limitations,airflow measurements with physical meters in air-handlingunits (AHUs) are not adequate or/and accurate for energy efficientoperations. Virtual airflow sensors that calculate airflowrate based on measurable operational data using fan curvesprovide a promising alternative. However, in-situ fan curves,rather than the manufacturer's fan curves, are needed foraccurate airflow calculations. This paper explores a comprehensiveprocedure for accurate in-situ fan curve calibration invariable-air-volume (VAV) systems by addressing three technicalchallenges. First, the sampling time is studied to minimizethe turbulence impact on the air velocity measurements.Second, for extended periods of airflow measurements using alimited number of velocity probes, the velocity distributionprofile across a traverse section is evaluated. Third, in orderto correct the flow rate measurement bias for volume trackingcontrols, a synchronized calibration between the supply airand return air fans is developed and studied. To validate thisprocedure, the in-situ fan head versus. airflow and efficiencyversus airflow curves for both the supply and return fans of atest AHU are calibrated and compared with the manufacturer'sfan curves. The investigation shows that the longersampling time for velocity measurements can compensate foraccuracy losses when the spatial traverse coverage is sacrificed.For example, for the 30 by 30 in. (0.76 by 0.76 m) ductin this study, the velocity mean by 25 sampling points on atraverse plane using a 3-second sampling time has an accuracyequivalent to that measured by three sampling points using a60-second sampling time. A coefficient of 1.276 is identified tocorrect the supply fan airflow measurements based on thereturn side through synchronized calibrations due to a lessideal traverse plane location in the supply duct. The largediscrepancy between the calibrated and manufacturer curvessuggests that in-situ fan curve calibration is necessary for theimplementation of virtual fan airflow sensors.