As consequence of increasing market volatility in chemical industry, plants should be designed for a certain flexible capacity. In order to meet fluctuating market demands a volume flexible plant with an increased capacity range may enable a more profitable production by avoiding a cost-intensive inventory of products. However, the profitability increase gained during production must be balanced with additional investment costs to build a plant with increased capacity range. During early project phases decision-making on such plant design requires a quantification of flexibility to evaluate the additional investment that may be spent for realization. This study presents a framework evaluating the additional investment that may be spent for a plant design with increased capacity range regarding volatile markets. A production planning method is integrated to determine sales and inventory costs adapting the production rate. Economic and investment risk analysis is applied to evaluate the economic improvement by additional volume flexibility in a volatile market. The maximum viable investment as a key performance indicator is introduced to evaluate the investment for increasing the capacity range economically. The framework is applied to a case study demonstrating its value during early design phases. (C) 2017 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.