When two or more ions contribute to the membrane potential across the plasma membrane (V_m) of a cell, it is likely that the membrane potential would not be at the equilibrium potential (V_eq.) for any of the contributing ions. Thus, no ion would be at its equilibrium (i.e., V_eq. ≠ V_m). When an ion is not at its equilibrium, an electrochemical driving force (V_DF) acts on the ion, causing the net movement of the ion across the membrane down its electrochemical gradient. The driving force is quantified by the difference between the membrane potential and the ion equilibrium potential (V_DF = V_m − V_eq.). The magnitude of the driving force indicates how far an ion is from its electrochemical equilibrium. The arithmetic sign (i.e., positive or negative) of the driving force acting on an ion along with the knowledge of the valence of the ion (i.e., cation or anion) can be used to predict the direction of ion flow across the plasma membrane (i.e., into or out of the cell). See Table 1 below, and the lecture notes on the Resting Membrane Potential for additional details.

Each calculator cell shown below corresponds to a term in the formula presented above. Enter appropriate values in all cells except the one you wish to calculate. Therefore, at least two cells must have values, and no more than one cell may be blank. The value of the blank cell will be calculated based on the other values entered. After a calculation is performed, the calculated cell will be highlighted and subsequent calculations will calculate the value of the highlighted cell (with no requirement to have a blank cell). However, a blank cell has priority over a highlighted cell.

Table 1 summarizes how the arithmetic sign of the driving force may be used to predict the direction of ion flow across the plasma membrane (i.e., into or out of the cell). For cations (positively charged ions such as Na⁺, K⁺, H⁺, and Ca²⁺), a positive driving force (i.e., V_DF > 0) predicts ion movement out of the cell (efflux) down its electrochemical gradient, and a negative driving force (i.e., V_DF < 0) predicts ion movement into the cell (influx). The situation is reversed for anions (negatively charged ions such as Cl⁻ and HCO₃⁻), where a positive driving force predicts ion movement into the cell (influx), and a negative driving force predicts ion movement out of the cell (efflux). If the membrane potential (V_m) is exactly at the equilibrium potential (V_eq.) for an ion, the driving force acting on the ion would be zero. If V_m = V_eq., it can be seen that V_DF = V_m − V_eq. = 0. In this case, the ion is said to be in electrochemical equilibrium, and there would be no net movement of the ion across the plasma membrane into or out of the cell (i.e., no net flux of ion).

See the lecture notes on the Resting Membrane Potential for additional details.