Two primary considerations for dairy barn ventilation are barn-level air exchange and the cow-level microclimate. Few studies and on-farm assessments address ventilation beyond limited description of fan presence and type of ventilation system. Our aim was to develop a standard method to characterize microenvironments and barn-level ventilation performance (for mechanical systems) to help dairy producers troubleshoot their ventilation systems. Forty-two barns (9, 5, and 28 cross-, tunnel-, and naturally ventilated, respectively) were evaluated in WI, MI, GA, TN, and TX to develop a novel airspeed mapping procedure. For barn-level ventilation performance, we measured barn and inlet dimensions, static pressure differential, temperature (T) and relative humidity (RH) at inlet and outlet, and fan number and models. To characterize microclimates at cow standing and resting heights (1.5 and 0.5 m, respectively), we measured T, RH, and airspeeds for 3 min/location. In mechanically ventilated barns, microclimate measurements to characterize the resting area and feed lane were taken in a representative half of the barn, divided along the direction of air flow; in naturally ventilated barns, these were taken in 1 pen per circulation fan configuration at every other stall for at least 2 fans/row. For each producer, we generated a report including barn description, ventilation performance, graphs of inlet and outlet T and RH over time, pen maps overlaid with microclimate measurements, an interpretive summary, and action items for potential improvements. In general, mechanically ventilated barns were consistent with industry standards, but 80% of barns had inconsistent airflow distribution resulting in some stalls with insufficient airspeeds (<1 m/s) at resting height. In naturally ventilated barns, 54 ± 30% of measured stalls had insufficient airspeeds at resting height, largely due to improper fan angle. 63% of all farms reported yearly fan maintenance. Our ventilation characterization method and novel airspeed maps provided dairy producers with customized feedback to improve their ventilation system performance.