Building air distribution terminal system designers and system installers require accurate quantitative information on the performance of the installed system to achieve optimum efficiency and levels of human comfort. This requires field installation adjustment values from published ideal pressure loss, air distribution and sound generation installation performance. This study documents the air output performance of different installation configurations of six types of ceiling diffusers and compares the results to performance when installed according to ANSI/ASHRAE Standard 70-2006. A diffuser inlet supply plenum was designed for optimum flow and was used to acquire a baseline set of data covering the six types of diffusers at different inlet neck sizes and inlet airflow rates. Full scale laboratory testing of typical field installation variations was completed for the same diffuser types and airflow rates with variations in damper installation, duct approach angle, duct type, duct vertical height above the diffuser and close coupling duct installation. A set of look-up tables were developed that can be used to easily predict how the installation configuration would affect diffuser performance compared to published data.