pKa Calculator (Ka ⇄ pKa ⇄ pKb, and acid/base forms at any pH)

Understanding the pKa calculator — concept and principle

This calculator converts between Ka, pKa, Kb and pKb for an acid and its conjugate base, and shows what share of the acid is in its acid and base forms at any pH. It is used in chemistry, biochemistry and pharmacy for buffers, titrations and drug ionisation.

At a pH above the pKa most of the acid has given up its proton.

How it is calculated

pKa = −log₁₀ Ka; Ka = 10^(−pKa)

Ka × Kb = Kw, so pKa + pKb = 14.00 at 25 °C

Henderson–Hasselbalch: [A⁻] ÷ [HA] = 10^(pH − pKa)

Share in base form = ratio ÷ (1 + ratio)

The principle

An acid HA gives up a proton to water, forming A⁻. Ka measures how far this goes; because Ka values span many powers of ten, chemists use the logarithmic pKa. The conjugate base's strength is tied to the acid's through the water equilibrium. Rearranging the Ka expression shows that the ratio of base to acid depends only on how far the pH is from the pKa: at pH = pKa they are equal, and each pH unit changes the ratio tenfold.

Worked example

Default inputs: Ka = 0.000018 (acetic acid), pH 5.

pKa = −log₁₀(1.8 × 10⁻⁵) = 4.7447.
pKb = 14 − 4.7447 = 9.2553; Kb = 5.556 × 10⁻¹⁰.
[A⁻]/[HA] = 10^(5 − 4.7447) = 1.8.
Base form = 1.8 ÷ 2.8 = 64.29%; acid form = 35.71%.

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

Value	pKa of the acid
4.5e-06	5.3468
9e-06	5.0458
1.35e-05	4.8697
1.8e-05	4.7447
2.25e-05	4.6478
2.7e-05	4.5686
3.6e-05	4.4437

How to read the result

A smaller pKa means a stronger acid. Buffers work best within about one pH unit of the pKa, where both forms are present in useful amounts. One pH unit above the pKa, about 91% is in the base form; two units above, about 99%.

Common mistakes

Forgetting that pKa + pKb = 14 holds only at 25 °C.
Mixing up the ratio direction ([A⁻]/[HA], not the reverse).
Using concentration instead of activity in concentrated solutions.

Keep in mind: Values assume dilute solutions at 25 °C. Temperature and ionic strength shift pKa.

Frequently asked questions

What is the pKa of acetic acid?

About 4.75.

When is an acid half dissociated?

When pH equals its pKa.

Why is pKa + pKb = 14?

Because Ka × Kb = Kw = 10⁻¹⁴ at 25 °C.

Is a lower pKa a stronger acid?

Yes.

Sources: OpenStax, Chemistry 2e (2019), sections 14.3 "Relative Strengths of Acids and Bases" and 14.6 "Buffers"

Checked 9 Nov 2026. Ka, pKa, Ka·Kb = Kw (pKw = 14.00 at 25 °C) and the Henderson–Hasselbalch relation: OpenStax, Chemistry 2e (2019), §14.3 “Relative Strengths of Acids and Bases” and §14.6 “Buffers” — standard results.

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