Acid pKa Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/8/2026
pKa is calculated with pKa = −log₁₀(Ka), where Ka is the acid's acidity constant. For acetic acid (Ka = 1.8×10⁻⁵), the pKa is about 4.74.
Explanation
The acidity constant Ka measures a weak acid's tendency to give up a proton in aqueous solution: the larger Ka is, the stronger the acid (it dissociates more). Because Ka values span many orders of magnitude (from 10⁻² to 10⁻¹⁰ for most common weak acids), chemists almost always use pKa, its negative decimal logarithm, to work with more readable and directly comparable numbers — exactly as pH turns H⁺ ion concentration into a more practical scale. An acid with a lower pKa is a stronger acid: acetic acid (pKa ≈ 4.74) gives up its proton more easily than phosphoric acid in its second acidity (pKa ≈ 7.2), which is a weaker acid. This pKa is the direct input to the Henderson-Hasselbalch equation (see our buffer solution pH calculator), which calculates the pH of a weak acid/conjugate base buffer mixture — this calculator answers the preceding step: how to obtain this pKa from the Ka constant measured experimentally or found in a reference table. A useful property to remember: when the concentration of the acid and its conjugate base are equal in a buffer solution, the pH of that solution is exactly equal to the acid's pKa — this is in fact the point where the solution's buffering capacity is at its maximum, which guides the choice of a suitable acid/base pair to buffer a solution at a given target pH.
Example: pKa of acetic acid
Inputs
Acidity constant Ka: 1.8×10⁻⁵ (0.000018).
Calculation
pKa = −log₁₀(0.000018) ≈ 4.74.
Result
Acetic acid has a pKa of about 4.74, a reference value used for example to prepare an acetate buffer.
Frequently asked questions
Why use pKa rather than Ka directly?
Because Ka values cover an extremely wide range, often expressed in scientific notation that is impractical to work with (1.8×10⁻⁵, 6.3×10⁻⁸...). pKa, by taking the negative decimal logarithm, brings these values down to a more compact and intuitive scale, generally between 0 and 14 for common weak acids — the same logic that motivated the introduction of pH for H⁺ ion concentration.
What does a negative or very low pKa mean?
A low or negative pKa corresponds to a high Ka (close to or above 1), meaning an acid that dissociates almost completely in solution — this is the definition of a strong acid (such as hydrochloric acid or sulfuric acid). These acids are generally considered fully dissociated in water, and their precise pKa has little practical use: it is precisely for these strongly dissociated solutions that our pH calculator is enough to directly describe the acidity, without going through pKa.
Does pKa depend on temperature?
Yes, like any equilibrium constant, Ka (and thus pKa) varies slightly with temperature, generally modestly for most common organic acids within the usual laboratory temperature range. Tabulated pKa values are generally given at 25°C; for rigorous accuracy at another temperature, corrected tables or the van't Hoff equation applied to the dissociation equilibrium should be used.