This paper describes an improved method for the calculation of sporadic-E signal strength which was mainly developed through the work of the Interim Working Party 6/8 of the CCIR. In this method, the effect of atmospheric radio refraction on the sporadic-E propagation path and of antenna height gain are given major consideration. Empirical formulas of sporadic-E attenuation are also presented for use in computer calculation. Finally, some examples are shown for comparing the values predicted by using this method with measured ones. This study is believed to be applicable to the lower VHF band and upper HF band (e.g., 10 to 150 MHz).