Ka to pKa Calculator
Convert between Ka, pKa, Kb, and pKb for an acid and its conjugate base.
Input type
Lower pKa means a stronger acid.
- 1
log₁₀(Ka)
log₁₀(1.80e-5) = -4.7447 - 2
pKa = −log₁₀(Ka)
−(-4.7447) = 4.74
How does this calculator work?
pKa is the negative base-10 log of Ka, so pKa = -log10(Ka) and Ka = 10^(-pKa). The conjugate base follows from Kw = 1e-14: Kb = Kw/Ka and pKb = 14 - pKa at 25 C. Enter any one of Ka, pKa, Kb, or pKb and the calculator returns the other three instantly.
Formula
How this is calculated
Pick which quantity you have using the Input type selector (Ka, pKa, Kb, or pKb) and enter its value. The acid dissociation constant Ka measures how completely an acid donates protons in water; pKa is simply its negative base-10 logarithm, pKa = -log10(Ka), so smaller pKa values correspond to stronger acids. The inverse relationship Ka = 10^(-pKa) lets you go back the other way.
The conjugate base is handled through the ion-product of water, Kw = 1e-14 at 25 C. Because Ka × Kb = Kw, the base constant is Kb = Kw/Ka, and taking logarithms gives the convenient sum rule pKa + pKb = 14, i.e. pKb = 14 - pKa. The calculator computes the remaining three quantities from whichever one you supply.
Ka and Kb are dimensionless equilibrium constants and must be positive, so logarithms are defined; pKa and pKb are pure numbers and may take any real value. All conjugate relationships assume aqueous solution at 25 C where Kw = 1e-14; at other temperatures Kw changes and the value 14 in pKa + pKb = 14 shifts accordingly.
Frequently asked questions
pKa = -log10(Ka). To reverse it, Ka = 10^(-pKa).
For a conjugate acid-base pair, Ka × Kb = Kw = 1e-14, so Kb = Kw/Ka and pKa + pKb = 14 at 25 C.
Yes. A smaller pKa corresponds to a larger Ka, meaning the acid dissociates more fully and is therefore stronger.
Also known as
TG we-Calculate Editorial Team. (2026). Ka to pKa Calculator [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/ka-to-pka-calculator
TG we-Calculate Editorial Team. "Ka to pKa Calculator." TG we-Calculate. 2026. https://we-calculate.com/calculator/ka-to-pka-calculator.
TG we-Calculate Editorial Team, "Ka to pKa Calculator," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/ka-to-pka-calculator
@misc{wecalculate_ka_to_pka_calculator, title = {Ka to pKa Calculator}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/ka-to-pka-calculator}}, year = {2026}, note = {TG we-Calculate} }
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