What this calculator does
The standard cell potential (E°) of an electrochemical cell only applies at standard conditions: 25°C, 1 atm pressure and 1 mol/L concentrations for everything involved in the reaction. The Nernst equation adjusts that standard potential to find the actual cell potential under any other conditions, using the reaction quotient Q to capture how far the actual concentrations differ from standard.
This calculator uses the full Nernst equation with temperature as a variable, rather than the simplified room-temperature-only version some textbooks quote. Entering 25°C reproduces the same result the simplified version gives, since that is exactly the temperature the simplified constant is built around.
The formula
The Nernst equation is E = E° − (RT/nF) × ln(Q), where R is the gas constant (8.314 J/(mol·K)), T is temperature in kelvin, n is the number of electrons transferred in the balanced half-reactions, F is the Faraday constant (96,485 C/mol), and Q is the reaction quotient describing the actual concentrations relative to standard.
| Term | Meaning |
|---|---|
| E° | The standard cell potential, measured at 25°C, 1 atm and 1 mol/L concentrations. |
| n | The number of electrons transferred in the balanced overall redox reaction. |
| Q | The reaction quotient, calculated the same way as an equilibrium constant expression but using the actual, non-equilibrium concentrations. |
The inputs explained
| Field | What to enter |
|---|---|
| Standard potential (E°) (V) | The standard cell potential for the reaction, usually found from a table of standard reduction potentials. |
| Temperature (°C) | The actual temperature the cell is operating at. |
| Electrons transferred (n) | The number of electrons transferred, taken from the balanced half-reactions. |
| Reaction quotient (Q) | The reaction quotient at the actual (non-standard) concentrations. |
When to use it
Finding a battery's actual voltage as it discharges
As a battery discharges, reactant concentrations fall and product concentrations rise, changing Q and, through the Nernst equation, gradually lowering the cell's actual voltage below its standard value.
Correcting for a non-standard temperature
A cell operating well above or below 25°C has a measurably different potential from its standard value, which the Nernst equation's temperature term accounts for directly.
Checking a concentration cell
A concentration cell has an E° of zero by definition (both electrodes are the same material), so its entire measured potential comes from the Nernst equation's concentration term alone.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does cell potential change as the reaction quotient Q moves away from 1, at 25°C with n=2?
The standard potential of a typical zinc-copper cell, adjusted for a range of reaction quotients.
Questions
What happens when Q equals 1?
The natural log of 1 is zero, so the entire correction term drops out and the cell potential equals the standard potential exactly. This is the standard-conditions case the Nernst equation reduces to.
Why does temperature affect cell potential?
Temperature appears directly in the RT/nF term, and it also affects reaction kinetics and equilibrium more broadly, but the equation here only captures the direct thermodynamic temperature dependence, holding the reaction quotient itself fixed.
What is a reasonable value for Q?
It depends entirely on the specific reaction and its actual concentrations, calculated the same way as an equilibrium expression using the real, non-equilibrium concentrations at the time. There is no universal typical value.
Is this the same as calculating equilibrium constant K?
No, though the two are related. At equilibrium, Q equals K, and the cell potential falls to zero at that point. This calculator finds potential at any given Q, not specifically at equilibrium.
For finding pKa from an equilibrium constant using a related logarithmic relationship, see the pKa from Ka calculator.