We present a measurement method for NH3 emissions from mechanically ventilated poultry buildings, which has been successfully used in a multistate, multidisciplinary research project to establish baseline values for the United States. To accurately determine building emission rate (ER, the product of pollutant concentration and exhaust airflow rate), accurate measurements must be made over representative periods and production phases. We present an innovative, low-cost, and accurate methodology that allows multiple buildings to be sampled in sequential order, provided that appropriate biosecurity measures are taken. Direct ventilation measurement is used in broiler houses, and in some layer operations, the number of fans is few enough that each fan can be individually calibrated in situ with a fan assessment numeration system device. For larger layer buildings with more than 15 fans, building ventilation is obtained from a tracer gas balance, utilizing CO2 generated by birds and feces. Other methods to determine building ventilation rate, which do not account for mechanical condition and degree of maintenance, both of which significantly affect actual fan capacity, introduce large errors. Twenty-eight portable monitoring units were fabricated and used for field acquisition of exhaust NH3 and CO2 concentrations and building static pressure. Ammonia is measured with redundant electrochemical sensors that are cyclically purged to eliminate errors caused by saturation from continuous exposure to air laden with NH3. The redundant NH3 unit minimizes missing data due to sensor failure.