It is very important in design problems of automatic production or assembly lines to estimate the fluctuation of daily output which results from the irregular occurrence of trouble and the variance of operational ratio of lines. An analytical method for the variance of operational ratio as the application of analysis for the operational ratio has been presented by previous papers. However, the method needs troublesome computations into practice. In compositions of lines, assembly stations are aligned in various ways, such as series, branched or parallel. Therefore, the method must be simple in calculation. In this report, a simple experimental equation which is given through simulations for production lines with various factors (trouble occurrence ratio, recovery times and buffer capacities) by a digital computer is presented. The presented equation gives appropriate results of variances of operational ratio in automatic production lines. Furthermore, a method to estimate fluctuations of daily products by the presented equation is shown with a numerical example.