Purpose – The Indonesian Islamic banking industry is very promising, but there has been no analysis of the optimal and decreasing growth rate of the industry. Information regarding these is essential for policy makers, Islamic bankers and all related parties to guide the future development of the industry and sets up proper plans and strategies. The paper aims to explore the optimal and decreasing growth rates of the industry and in so doing contribute to the current literature on the Indonesian Islamic banking industry. Design/methodology/approach – The paper first reports on the performance of the Indonesian Islamic banking industry, before explaining conditions where the Islamic banking industry is believed to be still immature. Third, in order to identify the optimal and decreasing growth rates, the paper estimates the future performance of the industry by using ARIMA models to identify periods where the growth rate is at optimal and decreasing points. Then, on the basis of a number of assumptions and statistical simulations, the analysis broadens to become qualitative in nature by determining the optimal numbers of Islamic banks to be established in the future. Findings – The paper generates some important findings. First, the optimum growth rate of the market share (0.12 percent) is predicted to occur in December 2012; the market share in that month is estimated to be 5.75 percent. Second, although the market share keeps growing, the paper finds the rate of increase to be slow, and in October 2018, it becomes negative, at 0.004 percent. The estimated market share in that time is 11.63 percent. Finally, the optimal number of Islamic banks in December 2012 is shown to be 23 and in October 2018, 24. Research limitations/implications – Qualitative information on the months of the optimal and decreasing growth rates and quantitative information on the optimal number of Islamic banks to be established are significant information for policy makers, Islamic bankers and other related parties. The information is likely to be important in relation to their efforts to develop the Islamic banking industry, to anticipate decreasing growth in the industry and to establish new Islamic banks. More generally, the paper helps the related parties to direct and guide the future development of the industry. Originality/value – To the best of the authors' knowledge, this is the first paper that attempts to establish optimal and decreasing growth rates in the Indonesian Islamic banking industry, or the optimal numbers of Islamic banks to be established in the future.