In this paper, we propose a new metric called "effective tunneling capacitance" (C_eff ^t) to quantify the transient swing in the gate leakage (gate oxide tunneling) current due to state transitions. C_eff ^t which is defined as the change in tunneling current with respect to the rate of change of input voltage is a unique metric and to our knowledge proposed here for the first time. This metric concisely encapsulates information about the swing in tunneling current during state transitions while simultaneously accounting for the transition rate and represents the capacitive load of the transistor due to tunneling. This capacitance can have impact on transistor characteristics being additive to its gate oxide and diffusion capacitances. We express C_eff ^t as functions of gate oxide thickness T_ox and on-chip power supply V_DD to make it useful for modeling in higher levels of design abstraction. We also statistically analyze the effects of process variations of T_ox and V_DD on its distribution