The present study was conducted to measure the total amount of annual rainfall all over the Bangladesh by different computational methods (Arithmetic average, Thiessen polygon and Isohyet). The rainfall data (1991-2011) of 34 rain gauges stations all over the Bangladesh were collected from Bangladesh Meteorological Department (BMD). Geographical Information System (GIS) tools were used in the present study to interpret the results. The Arc GIS 10.1 software (interpolation tool, Kringing) and different statistical analysis were carried out to get output results of different computational methods. GIS was used to show the patterns of rainfall all over the Bangladesh. The average rainfall of Bangladesh in the year of 1991 were 2876.029 mm, 2654.54 mm and 3066.47 mm by arithmetic average, thiessen polygon and isohyet method respectively whereas, in the year of 2011 the average rainfall by arithmetic average, thiessen polygon and isohyet methods were 2478.76 mm, 2213.046 mm and 2733.63911mm respectively. The annual average rainfall by all these three methods showed a decreasing trend science in 1991 to 2011 by considering all significant level of error in annual average rainfall that may be due to the physiographic stratification of the Bengal delta. The GIS interpolation tool (Kringing) also showed the decreasing patterns of annual average rainfall all over the Bangladesh. All these methods are not suitable to measure rainfall in different regions of the country. So, region-wise rainfall estimation must be done all over the country by these methods to find the actual scenario of rainfall patterns in Bangladesh.