Due to high installation costs and space limitations, airflow measurements with physical meters in air-handling units (AHUs) are not adequate or/and accurate for energy efficient operations. Virtual airflow sensors that calculate airflow rate based on measurable operational data using fan curves provide a promising alternative. However, in-situ fan curves, rather than the manufacturer's fan curves, are needed for accurate airflow calculations. This paper explores a comprehensive procedure for accurate in-situ fan curve calibration in variable-air-volume (VA V) systems by addressing three technical challenges. First, the sampling time is studied to minimize the turbulence impact on the air velocity measurements. Second, for extended periods of airflow measurements using a limited number of velocity probes, the velocity distribution profile across a traverse section is evaluated. Third, in order to correct the flow rate measurement bias for volume tracking controls, a synchronized calibration between the supply air and return air fans is developed and studied. To validate this procedure, the in-situ fan head versus, airflow and efficiency versus airflow curves for both the supply and return fans of a test AHU are calibrated and compared with the manufacturer's fan curves. The investigation shows that the longer sampling time for velocity measurements can compensate for accuracy losses when the spatial traverse coverage is sacrificed. For example, for the 30 by 30 in. (0.76 by 0.76 m) duct in this study, the velocity mean by 25 sampling points on a traverse plane using a 3-second sampling time has an accuracy equivalent to that measured by three sampling points using a 60-secondsampling time. A coefficient of 1.276 is identified to correct the supply fan airflow measurements based on the return side through synchronized calibrations due to a less ideal traverse plane location in the supply duct. The large discrepancy between the calibrated and manufacturer curves suggests that in-situ fan curve calibration is necessary for the implementation of virtual fan airflow sensors.