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pKa Calculator

Find pKa from an acid dissociation constant or from a buffer’s pH and conjugate-pair concentrations.

By ChemistryCalc•Updated October 28, 2026•References
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Results update as you type. Choose units beside a measurement; changing units converts the current value.

On this pageCalculatorHow to useFormula & methodWorked exampleA logarithmic view of acid strengthConcentrations are an approximation to activitiesFrequently asked questionsRelated calculatorsSources

01 Set up your calculation

  1. Choose the dissociation-constant method or buffer method.
  2. For a buffer, enter equilibrium concentrations for one conjugate acid/base pair.
  3. Interpret the inferred pKa at the temperature and conditions of the inputs.

02 The relationship

pKa = −log₁₀(Ka); for a buffer, pKa ≈ pH − log₁₀([A⁻]/[HA]).

For the assumptions behind these results, read the explanation below. You can also explore Hydrogen Ion Concentration and pH.

WORKED EXAMPLE

Follow the numbers

An acid with Ka = 1.8 × 10⁻⁵ has pKa = 4.74473. In the buffer mode, equal conjugate concentrations make pKa equal to the entered pH.

03 A logarithmic view of acid strength

Taking a negative base-10 logarithm turns small dissociation constants into convenient positive values. A tenfold larger Ka corresponds to a pKa smaller by one. Negative pKa values are possible and are not rejected merely because of their sign.

The buffer rearrangement uses the ratio of the conjugate base to the acid. Swapping that ratio changes the sign of the logarithmic correction, which is a common source of error.

04 Concentrations are an approximation to activities

The thermodynamic equilibrium constant is based on activities. Replacing activities with concentrations works best under conditions where nonideality is small or where a suitable conditional constant is being used.

A buffer estimate is not reliable if you enter total analytical concentrations that differ greatly from the equilibrium species concentrations. Polyprotic systems also need the correct acid/base pair for the dissociation step being examined.

Questions from the notebook

Does a smaller pKa mean a stronger acid?

For comparable conditions and the same dissociation convention, yes: a smaller pKa corresponds to a larger Ka.

Can one acid have more than one pKa?

Yes. An acid that donates more than one proton has separate dissociation steps with separate constants.

References & reading

Our explanations are written for ChemistryCalc. Read how we build and check our tools. These calculations are educational estimates where the underlying model requires assumptions.