Electrochemical capacitors can be divided into two types depending on whether the salt concentration in the electrolyte changes during charging and discharging. In the first type of capacitor, such as double-layer capacitors, the salt concentration in the electrolyte reduces during the charging of the capacitor. The maximum energy density of this type of capacitor will depend not only on the specific capacitance and the operating voltage, but also on the salt concentration of the electrolyte. In this paper, a formula describing the dependence of energy density on specific capacitance, operating voltage, and salt concentration is given based on the optimized weight (or volume) ratio of the electrode material and the electrolyte. It shows that for electrochemical capacitors using nonaqueous electrolytes, the maximum energy density of the capacitor will be limited mainly by the low salt concentrations ol the electrolyte. The relationship between the energy density and the mass density of the electrode is also given. The optimum mass density of the electrode can be obtained based on the value of the theoretical energy density for capacitors with different electrolytes. In the second type of capacitor, such as pseudocapacitors with metal oxide electrodes, the salt concentration in the electrolyte remains constant during charging and discharging. The maximum energy density of this type of capacitor will be Limited mainly by specific capacitance and operating voltage.