Nernst Potential Calculator
Calculate the equilibrium membrane potential for Na+, K+, Ca2+, Cl- and custom ions.
Enter intra- and extracellular concentrations and temperature in Celsius.
The Nernst equation calculates the equilibrium potential for a single ion across a membrane. This is the voltage at which the electrical force on the ion exactly cancels its concentration gradient, so the ion has no net tendency to move in either direction.
The Nernst equation:
E = (RT / zF) x ln([ion]_out / [ion]_in)
where R is the gas constant (8.314 J/mol K), T is temperature in Kelvin, z is the ion’s valence (charge), F is Faraday’s constant (96,485 C/mol), and the concentrations are extracellular and intracellular.
At 37 degrees Celsius this simplifies to approximately:
E = (61.5 / z) x log10([ion]_out / [ion]_in) mV
Typical equilibrium potentials in mammalian neurons:
- K+: E_K ~ -90 mV (high K+ inside, low outside)
- Na+: E_Na ~ +60 mV (high Na+ outside, low inside)
- Ca2+: E_Ca ~ +123 mV (dramatically higher outside)
- Cl-: E_Cl ~ -65 mV (valence is -1, which flips the sign)
The resting membrane potential of a neuron (-70 mV) is a weighted average of these, set primarily by resting K+ permeability. When an ion channel opens, the membrane potential is pulled toward that ion’s Nernst potential.
Understanding Nernst potentials is essential for interpreting action potentials, synaptic potentials, and the mechanisms of drugs that target ion channels such as local anesthetics, antiepileptics, and antiarrhythmics.