Buffer pH Calculator

Calculate buffer pH from dissociation constants and concentrations, or solve for any input.

Supports acid and base buffers with M, mM, and μM concentration units.

Updated September 2, 2026

CreatorFrank Zhao

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What is a buffer solution?

A buffer is a solution that resists changes in pH when small amounts of acid or base are added. It typically consists of a weak acid paired with its conjugate base (or a weak base paired with its conjugate acid). Buffers are essential in biochemistry, pharmaceutical formulation, environmental monitoring, and any application where a stable pH is critical.

This calculator uses the Henderson-Hasselbalch equation to compute the pH of a buffer from its dissociation constant and component concentrations. It works in both directions — enter any three known values and the fourth is solved automatically. Concentrations can be entered in molar ( $\mathrm{M}$ ), millimolar ( $\mathrm{mM}$ ), or micromolar ( $\mu\mathrm{M}$ ) units.

How to use this calculator

1Choose the buffer type: Acid (weak acid + conjugate base) or Base (weak base + conjugate acid).
2Enter the dissociation constant. For an acid buffer, that's Ka or pKa; for a base buffer, Kb or pKb. The two are converted automatically.
3Enter the concentration of the weak species and its conjugate. Any valid concentration unit (M, mM, or μM) works; the calculator handles conversions internally.
4The pH appears instantly. You can also work backwards — enter the pH and two other values to solve for the missing concentration or dissociation constant.

Worked example: acetic acid buffer

Acetic acid ( $\mathrm{CH_3COOH}$ ) has a pKa of 4.76. You prepare a buffer with 0.10 M acetic acid and 0.10 M sodium acetate.

\mathrm{pH} = \mathrm{pKa} - \log_{10}\!\left(\frac{[\mathrm{HA}]}{[\mathrm{A}^-]}\right)

\mathrm{pH} = 4.76 - \log_{10}\!\left(\frac{0.10}{0.10}\right)

=

4.76 - 0 = 4.76

When the acid and conjugate base concentrations are equal, the logarithm term vanishes and pH = pKa. This is a quick sanity check: if you see pH = pKa in the result, the two concentrations are balanced.

Formulas and variables

Acid buffer

\mathrm{pH} = \mathrm{pKa} - \log_{10}\!\left(\frac{[\mathrm{HA}]}{[\mathrm{A}^-]}\right)

$\mathrm{pKa}$ — negative logarithm of the acid dissociation constant: $\mathrm{pKa} = -\log_{10}(\mathrm{Ka})$

$[\mathrm{HA}]$ — concentration of the weak acid

$[\mathrm{A}^-]$ — concentration of the conjugate base

Base buffer

\mathrm{pH} = 14 - \mathrm{pKb} - \log_{10}\!\left(\frac{[\mathrm{B}^+]}{[\mathrm{BOH}]}\right)

$\mathrm{pKb}$ — negative logarithm of the base dissociation constant: $\mathrm{pKb} = -\log_{10}(\mathrm{Kb})$

$[\mathrm{BOH}]$ — concentration of the weak base

$[\mathrm{B}^+]$ — concentration of the conjugate acid

Bidirectional solving

Each formula works in four directions. In acid mode, you can solve for pH, pKa (or Ka), acid concentration, or salt concentration by supplying the other three values. The same applies to base mode with pH, pKb (or Kb), base concentration, and salt concentration. The dissociation constant conversions between K and pK are always available regardless of which other fields you fill in.

Frequently asked questions

How do I choose between entering Ka and pKa?

Use whichever value you have. If you enter Ka, the calculator instantly computes pKa (and vice versa) through the relation $\mathrm{pKa} = -\log_{10}(\mathrm{Ka})$ . The same applies to Kb and pKb. You only need to provide one.

Can I enter concentrations in different units?

Yes. Each concentration field supports M (molar), mM (millimolar), and μM (micromolar). You can mix units between the acid and salt fields — the calculator converts everything to molar internally before solving. For example, entering 100 mM for the acid and 0.1 M for the salt is the same as entering 0.1 M for both.

What does it mean when pH equals pKa?

When the concentrations of the weak acid and its conjugate base are equal, the log term in the Henderson-Hasselbalch equation becomes zero and $\mathrm{pH} = \mathrm{pKa}$ . This is the point of maximum buffer capacity — the solution is equally resistant to added acid and base.

Limitations

Ideal solution assumption. The Henderson-Hasselbalch equation treats concentrations as activities. At high ionic strength or concentrated solutions, activity coefficients can shift the effective pH by several tenths of a unit.
Temperature dependence. Ka and Kb are temperature-sensitive. The calculator uses the values you enter, so make sure they correspond to your actual experimental temperature (most published pKa values are at 25 °C).
pH range. Results are valid within 0–14. Values outside this range indicate conditions where the buffer model breaks down or the inputs are physically unrealistic.
Single equilibrium only. The calculator models one buffer system. Real solutions may contain multiple equilibria (e.g., carbonate + phosphate buffers), which require more complex calculations.

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