Ka to pKa Converter
The acid dissociation constant Ka quantifies how completely a weak acid ionizes in water. Because Ka values span more than twenty orders of magnitude, chemists prefer the logarithmic pKa scale, defined as the negative base-10 logarithm of Ka. This converter accepts either a Ka or a pKa value and returns the other, along with the conjugate base constants Kb and pKb at 25 C using the water relationship Ka times Kb equals Kw. A lower pKa means a stronger acid. Enter your value and pick which quantity you are supplying.
Ka and pKa formula
pKa = -log10(Ka)
Ka = 10^(-pKa)
pKb = pKw - pKa (pKw = 14.00 at 25 C)
Kb = 10^(-pKb)
Ka must be greater than zero. The conjugate base constants assume aqueous solution at 25 C, where Ka times Kb equals the ion product of water Kw.
Acid strength context
- A lower pKa indicates a stronger acid and a larger Ka.
- Strong acids such as hydrochloric acid have negative pKa values.
- Acetic acid has pKa 4.76, corresponding to Ka of 1.74 x 10^-5.
- The Henderson-Hasselbalch equation uses pKa directly to find buffer pH.
- For a conjugate pair, pKa plus pKb equals pKw, which is 14.00 at 25 C.
Ka and pKa: frequently asked questions
What is pKa?
pKa is the negative base-10 logarithm of the acid dissociation constant Ka: pKa = -log10(Ka). A smaller pKa means a stronger acid because it corresponds to a larger Ka and more complete dissociation. pKa is used because acid strengths span many orders of magnitude.
How do I convert pKa back to Ka?
Raise 10 to the power of the negative pKa: Ka = 10^(-pKa). For example, acetic acid has pKa 4.76, so Ka = 10^-4.76 = 1.74 x 10^-5. This converter performs both directions from a single Ka or pKa entry.
How are Ka and Kb related?
For a conjugate acid-base pair in water at 25 C, Ka x Kb = Kw = 1.0 x 10^-14, so pKa + pKb = 14.00. Knowing one constant gives the other for the conjugate partner. This converter reports the conjugate Kb and pKb automatically.
Is a higher or lower pKa a stronger acid?
A lower pKa indicates a stronger acid. Strong acids such as hydrochloric acid have negative pKa values, while weak acids such as acetic acid have pKa values around 4 to 5. Very weak acids have pKa values above 10.
Why use pKa instead of Ka?
Ka values range across more than 20 orders of magnitude, which is awkward to compare. The logarithmic pKa scale compresses these into convenient single- or double-digit numbers, making it easier to rank acid strength and to use the Henderson-Hasselbalch equation for buffers.
Official sources
- IUPAC Gold Book: acid dissociation constant and standard equilibrium constant.
- NIST Chemistry WebBook: thermochemical and equilibrium reference data.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.