Henderson-Hasselbalch Calculator

Buffer pH, and where the buffer stops buffering.

Buffer pH, and where the buffer stops buffering. pH = pKa + log([A⁻]/[HA]).

Written and maintained by Mohit PatelLast checked August 4, 2026How we build these

pKa

4.76 for acetic acid, 7.21 for the second proton of phosphoric acid.

Conjugate base [A⁻]

Weak acid [HA]

Buffer pH

4.76

Ratio 1 : 1 · within useful range

pH4.76

pKa4.76

Base : acid ratio1

Useful buffer range3.76 to 5.76

Buffering effectivelyYes

pH = pKa + log([A⁻]/[HA]). At equal concentrations the log term vanishes and the pH is the pKa exactly — which is also where the buffer is strongest, because adding acid or base moves the ratio least. Beyond a 10:1 ratio in either direction the solution has effectively stopped buffering, so choose a weak acid whose pKa sits within one unit of the pH you need rather than adjusting the ratio to breaking point.

How the Henderson-Hasselbalch Calculator works

pH = pKa + log([A⁻]/[HA]). At equal concentrations the log term vanishes and the pH is the pKa exactly, which is also where the buffer is strongest, because adding acid or base moves the ratio least.

Also known as: buffer ph calculator · buffer solution calculator · henderson hasselbalch equation · how to calculate buffer ph

Where to go next

The Henderson-Hasselbalch question rarely arrives on its own. These are the ones that usually come with it:

pOH Calculator — pOH, pH and both ion concentrations, with the 14 explained.
Weak Acid pH Calculator — Solved exactly, without the x ≪ C approximation.
Titration Calculator — Equivalence by equivalents, not by moles.
Speed Distance Time Calculator — Any one of the three from the other two, in four units.

Sources

Frequently asked questions

What is the Henderson-Hasselbalch equation?

pH = pKa + log([conjugate base]/[weak acid]). At a 1:1 ratio the pH equals the pKa.

What is the useful buffer range?

Roughly one pH unit either side of the pKa, corresponding to ratios from 1:10 to 10:1. Beyond that the solution has effectively stopped buffering.

How do I choose a buffer?

Pick a weak acid whose pKa is within one unit of the pH you need, rather than pushing an unsuitable one to an extreme ratio.

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