Informal economy is an observable issue around the globe. Researchers of both the developed and developing countries are continuously investigating its different dimensions (size, determinants, etc). The current study is an attempt to estimate the real size of informality (underground economy) in Pakistan. For its estimation the real factors of economy like employment level, political stability, tax to GDP ratio and cost of working in informal economy are used, over a period (Time series) of 1972 to 2010. HP-Prescott filter method is utilized for obtaining the potential GDP and actual GDP series from the estimated GDP which is estimated through Feasible Generalized Least Square (FGLS)1 and then from these series informal GDP is estimated. About 71 per cent of the informality was found, in the case of Pakistan economy. The finding suggests that combating informality, the factors taken must be targeted properly.