The paper focuses on the interaction between the solvency probability of a banking firm and the diversification potential of its asset portfolio when determining optimal equity capital. The purpose of this paper is to incorporate value at risk (VaR) into the firm-theoretical model of a banking firm facing the risk of asset return. Given the necessity to achieve a confidence level for solvency, we demonstrate that diversification reduces the amount of equity. Notably, the VaR concept excludes a separation of equity policy and asset-liability management.