Calculating contaminant concentrations in or the required ventilation for a space has been a difficult and confusing part in the application of the IAQ Procedure of ANSI/ASHRAE Standard 62.1-2004; Ventilation for Acceptable Indoor Air Quality. Appendix D of ASRAE Standard 62 presents one method for performing these calculations, but it is limited to the steady-state analysis of a single zone. More recently, two software tools have been developed by the United States National Institute of Standards and Technology (NIST) to facilitate these calculations and to include transient effects. One is the Indoor Air Quality Design Tool (IAQDT) that was developed to aid in contaminant-based design of ventilation systems, such as when using the IAQ Procedure. The IAQDT differs from Appendix D in that it does not assume steady state conditions to exist. The other is CONTAM, a multizone airflow and indoor air quality and ventilation analysis computer program designed to help predict airflows and contaminant concentrations in multizone building systems. This paper reports on the application of all three methods to a single zone, showing how the results obtained by each method may be similar, with exceptions occurring when transient effects are important. The paper also evaluates difficulties that arise from applying a single zone mass balance model to a multiple zone system. Application and appropriateness of each method for modeling indoor air quality and when using the IAQ Procedure will be discussed.