Solar radiation is the primary energy source for all physical and biochemical processes that take place on earth. Solar radiation level is some times recorded, but generally it needs to be estimated by empirical models based on frequently available meteorological records such as hours of sunshine, temperature or precipitation. The aim of this study was to determine the accuracy and applicability of a number of existing and newly developed formulae for calculating solar radiation from other weather variables. Data were taken from sixteen sites in different climatic zones in Iran. That had long term weather data sets, which included solar radiation. Coefficients for formulae were fitted using a two-year period of historic data for each site. Then the coefficients were used for estimation solar radiation for each site during the next five years. Correlation coefficients between values calculated for individual days and measured data and errors, were then computed.