STEMHub.tools
Buffers · Acids and bases

Buffer pH Calculator

Apply the Henderson–Hasselbalch equation to a weak-acid/conjugate-base buffer.

Formula shown Runs locally Reviewed Aug 11, 2026
Follow us on Google
Equation guide

Weak-acid/conjugate-base buffer approximation

Henderson–Hasselbalch equation

pH = pKa + log₁₀([A⁻]/[HA])

Equal-component condition

[A⁻] = [HA] ⇒ pH = pKa
Buffer calculator

Henderson–Hasselbalch equation

This is an equilibrium approximation for a weak-acid/conjugate-base buffer. [HA] is the weak acid and [A⁻] its conjugate base. Concentrations must be positive and in the same unit; the usual working range is 0.1 ≤ [A⁻]/[HA] ≤ 10.

pKa + log ratio

The estimated buffer pH will appear here.

Using the model

Symbols, assumptions and limitations

Interpretation notes

[HA] is the weak-acid concentration and [A⁻] is its conjugate-base concentration. The approximation is most reliable when both components are appreciable and 0.1 ≤ [A⁻]/[HA] ≤ 10; it uses concentrations in place of activities.

Worked examples

See the method in practice

01

Equal components

When [A−] = [HA], pH equals pKa.

02

Twice as much base

A base-to-acid ratio of 2 raises pH by log10(2), about 0.301.

Questions

Frequently asked

Can this calculate a strong-acid pH?

No. It is specifically for a weak acid and its conjugate base.

Should I use concentration or moles?

Concentrations may be replaced by mole amounts only when both components share the same final volume.

Sources & review

Equations and examples are checked against the references below. Results are educational and should be independently verified for safety-critical work.

OpenStax Chemistry — buffer solutions

Written by the STEM Hub editorial team · Reviewed August 11, 2026 · Review methodology