The determination of ammonia volatilization with sufficient spatial and temporal resolution requires a simple and versatile in situ measurement technique, particularly in developing countries. Therefore, a simple chamber method for determining ammonia (NH3) volatilization in the field (Dräger-Tube Method; DTM) was calibrated by comparison with simultaneous measurements with a micrometeorological Integrated Horizontal Flux (IHF) method. Five field experiments were conducted following urea fertilization on summer maize and winter wheat plots (1998–1999) at Fengqiu Experimental Station, Central China. The simplicity of the chamber method allowed for measurements to be carried out by trained farmers. The measurements with both methods yielded very similar patterns of NH3 fluxes and similar differences between fertilization treatments. Cumulative NH3 losses determined by the IHF method ranged from 14.6 to 47.9% and from 0.6 to 17.9% of urea-N applied for surface broadcast and incorporated fertilization, respectively. As expected, cumulated NH3 losses were underestimated by the DTM as compared to the IHF by about one order of magnitude. A calibration equation was calculated by multiple linear regression which included NH3 flux data as well as temperature and wind speed values. The calibration model yielded a modelling efficiency c2 of 0.86 resulting in an average estimation error of cumulative NH3 losses of 17%. The equation was validated by comparison of IHF measurements and DTM fluxes not considered in the derivation of the calibration formula. The calibration approach can be used under similar meteorological and field conditions irrespective of the soil characteristics or type of N fertilizer applied.