pKa Calculator — Ka to pKa, Kb to pKb and Back
By Santaji GadepatilinUpdated October 30, 2026
pKa is the negative logarithm of an acid's dissociation constant: the lower the pKa, the stronger the acid. This pKa calculator converts Ka to pKa (or pKa to Ka) and gives the Kb and pKb of the conjugate base — the numbers you need to pick a buffer, as in pH meter calibration buffers, or to estimate pH.
How to use the pKa calculator
Enter the acid dissociation constant Ka (you can type it as 0.000018 or 1.8e-5). The calculator returns the pKa and the Kb and pKb of the conjugate base. To go the other way, clear Ka and enter a pKa, Kb or pKb. For a weak base, enter its Kb in the Kb field and read the pKa of its conjugate acid.
Ka, pKa, Kb and pKb formulas
pKa = −log₁₀ Ka · Ka = 10−pKa
Ka × Kb = Kw = 1.0 × 10⁻¹⁴ · pKa + pKb = 14 (25 °C)
Example
Acetic acid, Ka = 1.8 × 10⁻⁵:
- pKa = −log₁₀(1.8 × 10⁻⁵) = 4.7447
- Kb (acetate ion) = 10⁻¹⁴ ÷ 1.8 × 10⁻⁵ = 5.556 × 10⁻¹⁰
- pKb = 14 − 4.7447 = 9.2553
pKa of common weak acids at 25 °C
| Acid | pKa | Ka | pKb of conjugate base | Kb of conjugate base |
|---|---|---|---|---|
| Phosphoric acid (pKa₁) | 2.15 | 7.08 × 10⁻³ | 11.85 | 1.41 × 10⁻¹² |
| Hydrofluoric acid | 3.17 | 6.76 × 10⁻⁴ | 10.83 | 1.48 × 10⁻¹¹ |
| Formic acid | 3.75 | 1.78 × 10⁻⁴ | 10.25 | 5.62 × 10⁻¹¹ |
| Lactic acid | 3.86 | 1.38 × 10⁻⁴ | 10.14 | 7.24 × 10⁻¹¹ |
| Benzoic acid | 4.20 | 6.31 × 10⁻⁵ | 9.80 | 1.58 × 10⁻¹⁰ |
| Acetic acid | 4.76 | 1.74 × 10⁻⁵ | 9.24 | 5.75 × 10⁻¹⁰ |
| Carbonic acid (pKa₁) | 6.35 | 4.47 × 10⁻⁷ | 7.65 | 2.24 × 10⁻⁸ |
| Hypochlorous acid | 7.53 | 2.95 × 10⁻⁸ | 6.47 | 3.39 × 10⁻⁷ |
| Hydrocyanic acid | 9.21 | 6.17 × 10⁻¹⁰ | 4.79 | 1.62 × 10⁻⁵ |
| Ammonium ion | 9.25 | 5.62 × 10⁻¹⁰ | 4.75 | 1.78 × 10⁻⁵ |
Literature values vary by about ±0.05 between sources and with ionic strength.
Notes
- Kw changes with temperature (pKw ≈ 14.17 at 20 °C and 13.26 at 50 °C), so pKa + pKb = 14 holds only at 25 °C.
- Polyprotic acids have one pKa per proton — phosphoric acid has 2.15, 7.20 and 12.35.
- Use the Henderson–Hasselbalch calculator to find buffer pH from pKa and the acid–base ratio.
Frequently asked questions
How do I convert Ka to pKa?
pKa = −log₁₀(Ka). Acetic acid has Ka = 1.8 × 10⁻⁵, so its pKa is 4.74.
How do I convert pKa back to Ka?
Ka = 10^(−pKa). Clear the Ka field, enter the pKa and the calculator does the antilog.
What is the relationship between pKa and pKb?
For a conjugate acid–base pair in water at 25 °C, Ka × Kb = Kw = 1.0 × 10⁻¹⁴, so pKa + pKb = 14.
What does pKa tell you about a buffer?
A buffer works best within about ±1 pH unit of the acid's pKa, and at pH = pKa the acid and its conjugate base are present in equal amounts.