Buffer pH Calculator (Henderson-Hasselbalch)

Understanding the buffer pH calculator — concept and principle

A buffer is a mixture of a weak acid and its conjugate base (or a weak base and its conjugate acid) that resists changes in pH when small amounts of acid or base are added. This calculator uses the Henderson–Hasselbalch equation to find the pH of a buffer from its pKa and the two concentrations, and also shows the base-to-acid ratio you would need for pH 7 with the same pKa.

Three parts base to two parts acid nudges the pH 0.176 above the pKa, to 4.94.

How it is calculated

pH = pKa + log₁₀([A⁻] ÷ [HA])

pKa: acid dissociation constant of the weak acid, as −log₁₀ Ka.
[HA]: concentration of the acid form; [A⁻]: concentration of the conjugate base. Any unit works as long as both are the same, because only the ratio matters.
Ratio for a target pH: [A⁻] ÷ [HA] = 10^(pH_target − pKa), for the target pH you enter (default 7).

The principle

Start from the equilibrium constant Ka = [H⁺][A⁻] ÷ [HA]. Taking −log₁₀ of both sides and rearranging gives pH = pKa + log([A⁻]/[HA]). When acid is added, A⁻ absorbs the H⁺ and becomes HA; when base is added, HA gives up H⁺. The ratio changes only slightly, so the pH barely moves. The equation assumes that the amounts of HA and A⁻ in solution equal the amounts you added, which holds when both are much larger than [H⁺] and [OH⁻].

Worked example

Defaults: pKa = 4.76 (acetic acid), [HA] = 0.1, [A⁻] = 0.15.

Ratio: 0.15 ÷ 0.1 = 1.5.
log₁₀(1.5) = 0.176091.
pH = 4.76 + 0.176091 = 4.936091259.
Ratio for pH 7: 10^(7 − 4.76) = 10^2.24 = 173.7800829.

Each point is the calculator re-run with only “pKa” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

pKa	pH
1.19	1.366091259
2.38	2.556091259
3.57	3.746091259
4.76	4.936091259
5.95	6.126091259
7.14	7.316091259
9.52	9.696091259

How to read the result

When [A⁻] = [HA], pH equals pKa, and the buffer resists change best. A buffer works well within about pKa ± 1, i.e. base-to-acid ratios between 0.1 and 10. The pH 7 ratio of about 174 shows that an acetate buffer is a poor choice at pH 7; you would choose an acid whose pKa is near 7, such as the phosphate pair H₂PO₄⁻/HPO₄²⁻ (pKa about 7.2). Higher total concentration gives more buffer capacity but does not change the pH.

Common mistakes

Putting the acid on top of the ratio, which flips the sign of the log term.
Using pKa values from a different temperature or ionic strength than your solution.
Expecting a buffer to hold its pH far outside pKa ± 1.
Using the equation for very dilute buffers, where water and dissociation shift the real pH.

Keep in mind: Henderson–Hasselbalch uses concentrations instead of activities. In real biological and analytical buffers, ionic strength and temperature can shift the pH by a tenth of a unit or more, so always confirm with a calibrated pH meter.

Frequently asked questions

What is the best pH range for a buffer?

Within about one pH unit of the pKa, with the best capacity at pH = pKa.

Can I use moles instead of concentrations?

Yes. Both species are in the same volume, so the mole ratio equals the concentration ratio.

Does diluting a buffer change its pH?

Ideally no, because the ratio stays the same; in practice moderate dilution shifts it slightly and capacity drops.

Sources: Harris, D. C., Quantitative Chemical Analysis, W. H. Freeman · Atkins, P., de Paula, J., Physical Chemistry, Oxford University Press

Checked 8 Nov 2026. Standard textbook formulas in SI units, computed in your browser. For learning and estimates — real designs must follow your local codes and be checked by a qualified engineer.

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