Healthy indoor air quality is a basic standard for good living environment, and air cleaners are commonly used in residential applications, especially in China, to control indoor PM_2.5 pollutions from outdoors. An accurate cleaner selection method will help keep satisfying indoor air quality, as well as reducing cost and materials of equipment. Three key parameters, namely, air exchange rate (a), particle penetration factor (P) and deposition rate (k), have been suggested as direct influence on air cleaner selection when pollutants are coming from building infiltration. In relevant standards, however, there is no method that can link the three key parameters to various airtightness levels of external windows, which may often exist in real applications. This study, therefore, has proposed a calculation method for deciding a, P and k, based on monitored indoor and outdoor PM_2.5 mass concentrations, with given recommended design values under different external window airtightness levels. Results showed that both air exchange rates and penetration factors were significantly influenced by the airtightness level of windows, and both their values were found to be different from the recommended values in the current Chinese standard, providing evidence to support future standard revision and update. Meanwhile, deposition rates showed good agreement with the recommended value in the current Chinese standard (i.e. 0.2h⁻¹). Additionally, with increased window airtightness level, the required clean air delivery rate (CADR) of air cleaners showed a downward trend, which means an air cleaner with smaller size, lower energy consumption and less material.