The Nernst equation calculates the equilibrium potential (also referred to as the Nernst potential) for an ion based on the charge on the ion (i.e., its valence) and its concentration gradient across the membrane. Temperature also influences the Nernst potential (see Nernst equation below). A Nernst potential will develop across a membrane if two criteria are met: (1) if a concentration gradient exists across the membrane for a given ion, and (2) if selective permeation pathways (i.e., selective ion channels) exist that allow transmembrane movement of the ion of interest. Finally, for selective ion channels, where the selectivity filter strongly favors the permeation of one ion over other ions, the Nernst potential also predicts the reversal potential (V_rev) of the current-voltage (I-V) relationship.

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 four 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.

As mentioned above, the Nernst equation calculates the equilibrium potential (V_Eq.) for an ion based on the charge on the ion (i.e., its valence) and its concentration gradient across the membrane. If only one ionic species is present in the system and/or only one type of ion channel is present, the Nernst potential also determines the resulting membrane potential (V_m). Of course, it must be assumed that the ion channels are open in order to allow transmembrane movement of the ionic species for which the channel is specific. If channels for two or more ions are present (and are open), both ions contribute to the membrane potential. In this case, V_m will not be at the equilibrium potential for either ion and, thus, no ion will be at equilibrium. Instead, the value of V_m will be determined by the relative contribution of each ion to the membrane potential. The relative contribution of each ion is governed not only by its concentration gradient and valence, but also by its relative permeability. Membrane permeability for an ion is defined as the relative ease with which the ion crosses the membrane, and is proportional to the total number of open channels for that ion in the membrane. To calculate the membrane potential when two or more ions play a role, use the Goldman-Hodgkin-Katz (GHK) equation.