Indoor air quality is affected by source strength of pollutants, ventilation rate, decay rate, outdoor level, and so on. Although technologies measuring these factors exist directly, direct measurements of all factors are not always practical in most field studies. The purpose of this study was to develop an alternative method to estimate these factors by application of multiple measurements. For the total duration of 30 days, daily indoor and outdoor $NO_2$ concentrations were measured in 30 houses in Brisbane, Australia, and for 21 days in 40 houses in Seoul, Korea, respectively. Using a box model by mass balance and linear regression analysis, penetration factor (ventilation divided by sum of air exchange rate and deposition constant) and source strength factor (emission rate divided by sum of air exchange rate and deposition constant) were calculated, Sub-sequently, the ventilation and source strength were estimated. In Brisbane, the penetration factors were $0.59pm0.14$ and they were unaffected by the presence of a gas range. During sampling period, geometric mean of natural ventilation was estimated to be $l.l0pm1.5l$ ACH, assuming a residential $NO_2$ decay rate of 0.8 hr^{-1}$ in Brisbane. In Seoul, natural ventilation was $1.15pm1.73$ ACH with residential $NO_2$ decay rate of 0.94 hr^{-1}$ Source strength of $NO_2$ in the houses with gas range $(12.7pm9.8$ ppb/hr) were significantly higher than those in houses with an electric range $(2.8pm2,6$ ppb/hr) in Brisbane. In Seoul, source strength in the houses with gas range were $l6.8pm8.2$ ppb/hr. Conclusively, indoor air quality using box model by mass balance was effectively characterized.