Nernst Equation Equilibrium Potential Calculator

Calculator Inputs

Ion Name

Ion Charge Number

Temperature

Temperature Unit

Inside Concentration

Inside Unit

Outside Concentration

Outside Unit

Membrane Voltage Vm

Decimal Precision

Quick Ion Preset

Report Notes

Formula Used

The calculator uses the Nernst equation for one ion:

E = (R × T) / (z × F) × ln(Cout / Cin)

Where E is equilibrium potential in volts, R is the gas constant, T is Kelvin temperature, z is ion charge, F is Faraday constant, Cout is outside concentration, and Cin is inside concentration.

For millivolts, the calculated voltage is multiplied by 1000. The tenfold slope is calculated with 2.303RT / zF.

How to Use This Calculator

Enter the ion name, such as potassium, sodium, chloride, or calcium.
Enter the ion charge. Use negative charge for anions.
Add inside and outside concentrations with matching or different units.
Choose the temperature unit and enter the temperature value.
Add membrane voltage if you want driving force.
Choose decimal precision and press the calculate button.
Review the result above the form.
Download the CSV or PDF report when needed.

Example Data Table

Ion	Charge	Outside	Inside	Temperature	Approximate Potential
Potassium K+	+1	5 mM	140 mM	37 °C	-89.0 mV
Sodium Na+	+1	145 mM	12 mM	37 °C	66.6 mV
Chloride Cl-	-1	110 mM	10 mM	37 °C	-64.1 mV
Calcium Ca2+	+2	1.8 mM	0.0001 mM	37 °C	130.9 mV

Understanding Equilibrium Potential

Equilibrium potential is the voltage that balances chemical movement and electrical force for one ion. It is also called reversal potential in many cell studies. The Nernst equation predicts this value from temperature, ion charge, and concentration ratio. The result helps explain why sodium, potassium, chloride, and calcium create different membrane signals.

Why This Calculator Helps

Manual work can be slow because units and logarithms must stay consistent. This tool converts concentration units to molar values before it finds the ratio. It also converts temperature to Kelvin. That keeps the gas constant, Faraday constant, and formula aligned. You can enter common laboratory units, select precision, and add membrane voltage for a driving force check.

Interpreting the Result

A positive equilibrium potential means the outside to inside gradient favors a positive voltage for that ion at balance. A negative value means balance occurs at a negative voltage. Charge matters strongly. Chloride often uses a negative charge, so its sign may reverse compared with a cation using the same ratio. Very small inside or outside values can produce large potentials, so measured concentrations should be realistic.

Practical Use Cases

Researchers use Nernst values while reading patch clamp data. Students use them when learning membrane physiology. Chemists use the same thermodynamic idea for electrochemical gradients. The calculator is useful for comparing several ions under one temperature. It also helps when reports need clear steps, formula details, and downloadable records.

Accuracy Tips

Use activity values when high accuracy is required. Concentrations are practical, but ion activity can differ in concentrated solutions. Use the same compartment convention for every calculation. This page uses outside divided by inside. Confirm the ion charge before submitting. Sodium and potassium are usually plus one. Calcium is plus two. Chloride is minus one.

Final Notes

The Nernst equation gives an equilibrium value for one ion only. Real cells contain many ions and open channels. Their actual membrane voltage may differ from one Nernst value. Still, this calculation gives a strong baseline. It turns concentration data into a clear voltage estimate for study, teaching, and quick laboratory review. It supports repeatable comparisons during lessons, audits, and experiment planning.

FAQs

What does equilibrium potential mean?

It is the voltage where one ion has no net movement across a membrane. Chemical diffusion and electrical force are balanced at this value.

Which concentration goes on top?

This calculator uses outside concentration divided by inside concentration. Keep this convention consistent when comparing different ions.

Can I use millimolar values?

Yes. You can use M, mM, µM, or nM. The calculator converts each selected unit before applying the Nernst equation.

Why is ion charge important?

Ion charge changes the sign and size of the potential. Chloride commonly uses -1, while sodium and potassium commonly use +1.

Does temperature affect the result?

Yes. Higher Kelvin temperature increases the voltage factor. Always enter the temperature that matches your experiment or example.

What is driving force?

Driving force is membrane voltage minus equilibrium potential. It shows how far the membrane voltage is from that ion balance point.

Can this calculator handle calcium?

Yes. Enter calcium charge as +2. Use realistic outside and inside values because calcium gradients can be very large.

Is this the same as membrane potential?

No. This result is for one ion. Real membrane potential depends on several ions, permeability, pumps, and channel activity.