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Ka to pKa Calculator

The pKa of an acid, so weak and strong acids compare on one scale.

How it is calculated

Enter the acid dissociation constant Ka as a plain number. The tool returns pKa = −log10(Ka), rounded to two decimals. Ka is dimensionless here (the molar concentration units are dropped, as in most tables) and must be strictly greater than zero, because the logarithm of zero or a negative number does not exist. Very small values are most easily typed in scientific notation: 1.8e-5 means 1.8 × 10-5 and so does 0.000018. A text such as 1.8×10^-5 is not understood. Each tenfold increase in Ka lowers pKa by exactly 1, so a Ka of 1 gives 0.00 and a Ka above 1 gives a negative pKa, which is allowed for strong acids. The tool converts one value only and does not pick the right pKa for polyprotic acids.

Worked example

Acetic acid has Ka = 1.8 × 10-5, typed as 1.8e-5. Split the logarithm: log10(1.8 × 10-5) = log10(1.8) + (−5) = 0.2553 − 5 = −4.7447. Changing the sign gives pKa = 4.7447, which the tool shows as 4.74. As a sense check, Ka lies between 10-5 (pKa 5) and 10-4 (pKa 4), so a pKa between 4 and 5 is right.

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Common mistakes

  • Typing the exponent as a separate number, such as 1.8 and then 5. The field takes the whole value: write 1.8e-5 or 0.000018, not 1.8 (which would give a pKa near −0.26).
  • Forgetting the minus sign in the exponent. 1.8e5 is a huge Ka and gives a negative pKa, whereas 1.8e-5 is the weak-acid value you usually mean.
  • Believing that a larger pKa means a stronger acid. It is the opposite: a smaller pKa corresponds to a larger Ka and a stronger acid.
  • Confusing Ka with Kb of the conjugate base. A base's Kb converts to pKb; to get the pKa of its conjugate acid use pKa + pKb = 14 at 25 °C, which this tool does not do.
  • Rounding Ka first and then converting. A pKa read to two decimals can differ in the second decimal if Ka was shortened beforehand.

About this calculator

The acid dissociation constant (Ka) measures how completely a weak acid ionizes in water — a larger Ka means a stronger acid. Because Ka values span many orders of magnitude (from 10⁻¹ down to 10⁻¹⁴ or smaller), chemists convert them to a logarithmic scale using pKa = −log₁₀(Ka), the same transform used for pH. On this scale, a lower pKa indicates a stronger acid, and the values are compact enough to compare and tabulate easily.

Chemistry students and instructors use pKa constantly when studying acid-base equilibria, buffer solutions, and the Henderson-Hasselbalch equation, while pharmaceutical chemists rely on it to predict how a drug's ionization state — and therefore its solubility and absorption — changes with pH. Analytical and organic chemists likewise reference pKa tables to choose appropriate buffers or predict reaction behavior.

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