The fine particulate matter, generally known as PM2.5, has great impact on the air quality and human health. Although closing external windows can help prevent outdoor PM2.5 going into indoors, many studies have shown that a significant number of particles can still pass the building façade through the cracks around the window. In order to quantify the influence of the external window crack structure and some relevant parameters, such as room dimension, on the indoor PM2.5 mass concentration, this paper introduces an updated model from a previously published paper by the authors . The model was developed based on two-month field measured data from five unoccupied offices located in the central area of Beijing (capital city located in northern China), and then was validated against a new dataset measured in Guangzhou (a major city located in southern China). The model can be used to quantify the indoor PM2.5 mass concentration based on the instant outdoor PM2.5 level, considering influences from external window crack structure, room dimension and outdoor meteorological conditions, i.e. outdoor wind speed and relative humidity.