Enter the pH value, the concentration of the conjugate base, and the concentration of the weak acid into the pKa Calculator. The calculator will evaluate the pKa. 

pKa Formula

The following formula is used to calculate the pKa. 

pKa = pH - Log(CB/WA)


  • pKa is the pKa ( )
  • pH is the pH value 
  • CB is the concentration of conjugate base 
  • WA is the concentration of weak acid 

To calculate pKa, subtract the value of the log of the conjugate base concentration divide by the weak acid concentration from the pH value.

How to Calculate pKa?

The following steps outline how to calculate the pKa.

  1. First, determine the pH value.
  2. Next, determine the concentration of conjugate base. 
  3. Next, determine the concentration of weak acid. 
  4. Next, gather the formula from above = pKa = pH – Log(CB/WA).
  5. Finally, calculate the pKa.
  6. After inserting the variables and calculating the result, check your answer with the calculator above. 

Example Problem:

Use the following variables as an example problem to test your knowledge.

pH value = 5

concentration of conjugate base = .32

concentration of weak acid = .45

pKa = pH – Log(CB/WA) =  ?