The Nernst equation is a mathematical approximation that allows scientists to predict the charge and voltage in an electrochemical system. Biologists use the Nernst equation to calculate the voltage across cell membranes. The Nernst equation uses information about different ions in the cell and their concentrations to calculate the cell's membrane voltage. This is important because a cell's voltage can give doctors important information about the health of a cell.
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Write down all the information that the problem gives you. If you are being asked to calculate the cell's voltage based on a particular ion, the problem should give you information on the concentration of that ion both inside and outside of the cell.

Look up the values of the relevant constants in the Nernst equation. This equation requires you to know the temperature at which the reaction takes place, which usually is human body temperature if you are calculating cell voltage. "R" represents the universal gas constant, which is 8.314; "F" stands for Faraday's constant, which is 96485. Lastly, the charge of the ion in question is always constant, and is specific to the ion that you are working with. You may need to look up the charge for less common ions.

Multiply the universal gas constant by the temperature of your reaction; record your answer.

Multiply the charge of the ion by Faraday's constant; record this answer as well.

Divide the number that you got in Step 3 by the number that you got in Step 4, and record this number.

Divide the concentration of the ion outside of the cell by the concentration of the ion inside the cell. Make sure that you do not switch these numbers, or the voltage that you calculate will be incorrect.

Take the natural log of the number that you obtained for in Step 6. The natural log is usually abbreviated "ln" on a calculator; you should be able to find this button on most scientific calculators.

Multiply the answer you got in Step 7 by the number that you got in Step 5 to get your final answer for cell membrane voltage. Check your answer to make sure that this is a reasonable number for a cellular voltage. Most cell voltages are between 100 and 100 millivolts.