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Chemistry formula

How to calculate henderson-Hasselbalch equation

The Henderson-Hasselbalch equation estimates buffer pH from a weak acid's pKa and the conjugate base-to-acid ratio.

See the formulaOpen calculator
Direct answer
Formula
pH = pKa + log10([A-] / [HA])

Use the table below to match each symbol with the right input. Keep units consistent before you start.

Henderson-Hasselbalch equation formula
Start here when you only need the equation and variable names.
Formula
pH = pKa + log10([A-] / [HA])

The Henderson-Hasselbalch equation estimates buffer pH from a weak acid's pKa and the conjugate base-to-acid ratio.

Variable meanings
Check each symbol, meaning, and unit before you calculate.
SymbolNameDescriptionUnit
pHBuffer pHEstimated acidity of the buffer solution.Depends on inputs
pKaAcid dissociation constantpH where weak acid and conjugate base are present equally.Depends on inputs
[A-]Conjugate baseConcentration or comparable amount of conjugate base.Depends on inputs
[HA]Weak acidConcentration or comparable amount of weak acid.Depends on inputs
When to use this formula
Check that your situation matches the formula before you trust the result.
  • Use it for buffer systems where a weak acid and its conjugate base are present in meaningful, comparable amounts.
Step-by-step method
Follow these steps when you are solving it by hand.
  1. 1Use matching units for conjugate base and weak acid concentration or amount.
  2. 2Divide conjugate base by weak acid to get the base-to-acid ratio.
  3. 3Take the base-10 logarithm of that ratio.
  4. 4Add the result to pKa.
  5. 5Check that the desired pH is reasonably close to the buffer pKa.
Examples
These sample numbers show the order of operations and units.
Equal acid and base
pKa
4.76
[A-]
0.10 M
[HA]
0.10 M
  1. 1.pH = 4.76 + log10(0.10 / 0.10)
  2. 2.pH = 4.76 + log10(1)
  3. 3.pH = 4.76
Result
Equal acid and base amounts give pH equal to pKa.
Target pH ratio
pKa
4.76
pH
5.00
  1. 1.[A-] / [HA] = 10^(pH - pKa)
  2. 2.[A-] / [HA] = 10^(5.00 - 4.76)
  3. 3.[A-] / [HA] = 1.74
Result
A pH of 5.00 needs about 1.74 parts conjugate base per 1 part acid.
Mistakes to avoid
Small input or unit errors can change the answer a lot.
  • Using zero or negative acid/base amounts, which cannot go inside the logarithm.
  • Swapping acid and conjugate base in the ratio.
  • Expecting a buffer to work well far outside about one pH unit from pKa.
  • Applying the equation to strong acid or strong base neutralization without buffer assumptions.
Open the Buffer Solution Calculator
Open the calculator for the answer without rewriting the formula.

Enter your values in the related calculator, then compare the output with the hand method above.

Open Buffer Solution Calculator
FAQs
Short answers for common formula questions.
Can I calculate Henderson-Hasselbalch equation by hand?

Yes. Write the units next to each value, convert rates or measurements first, and round only the final answer.

Why does my Henderson-Hasselbalch equation result differ from another calculator?

Most differences come from rounding, unit conversions, rate timing, or a slightly different version of the formula.

When is the Buffer Solution Calculator better than hand math?

Open the Buffer Solution Calculator to check several scenarios or skip the hand arithmetic.