Ka to pKa Calculator
The acid dissociation constant Ka measures how completely an acid splits into ions in water, but its values span many orders of magnitude and are awkward to compare directly. A weak acid might have a Ka of 0.000018 while a stronger one is hundreds of times larger, and reading those tiny fractions side by side is error-prone. The pKa scale fixes this by taking the negative base-10 logarithm of Ka, compressing the whole range into a tidy number that is usually between about -2 and 16. A lower pKa means a stronger acid; a higher pKa means a weaker one, and each whole-number step represents a tenfold change in Ka. This calculator takes a Ka value, including one written as a decimal, and returns the pKa to two decimal places. The conversion is the standard relationship used throughout acid-base chemistry and is the same one underpinning the Henderson-Hasselbalch equation for buffers. Reference Ka and pKa values for common acids are published by the National Institute of Standards and Technology. Every figure here is computed deterministically from the formula pKa = -log10(Ka), shown in full below, with a worked example that reconciles exactly to the calculator so you can check each step for yourself.
The pKa is the negative base-10 logarithm of the acid dissociation constant: pKa = -log10(Ka). For acetic acid with a Ka of 1.8 x 10^-5, the pKa is 4.74. A lower pKa means a stronger acid.
Ka to pKa formula
pKa = -log10(Ka)
Ka = acid dissociation constant
log10 = base-10 logarithm
inverse: Ka = 10^(-pKa)
The base-10 logarithm of a small fraction is negative, so negating it produces a positive pKa for most weak acids. Each unit decrease in pKa corresponds to a tenfold increase in Ka, which is why the pKa scale makes acid strengths easy to compare.
Worked example
Acetic acid has an acid dissociation constant Ka of 1.8 x 10^-5, written 0.000018.
- Identify Ka: Ka = 0.000018
- Take the base-10 logarithm: log10(0.000018) = -4.7447
- Negate the result: pKa = -(-4.7447) = 4.7447
- Round to two decimal places: pKa = 4.74
So acetic acid has a pKa of about 4.74. This is the calculator's default input, so the result above matches the widget exactly.
Ka and pKa of common acids
Reference values at 25 degrees Celsius. A smaller pKa means a stronger acid.
| Acid | Ka | pKa |
|---|---|---|
| Hydrofluoric acid | 6.6 x 10^-4 | 3.18 |
| Acetic acid | 1.8 x 10^-5 | 4.74 |
| Carbonic acid | 4.3 x 10^-7 | 6.37 |
| Ammonium ion | 5.6 x 10^-10 | 9.25 |
| Bicarbonate ion | 4.7 x 10^-11 | 10.33 |
Reference constants: US National Institute of Standards and Technology (NIST).
Ka to pKa calculator: frequently asked questions
How do you convert Ka to pKa?
Take the base-10 logarithm of the acid dissociation constant Ka and negate it: pKa = -log10(Ka). For example, an acetic acid Ka of 1.8 x 10^-5 gives pKa = -log10(0.000018) = 4.74. The logarithm compresses a very small Ka into a convenient number that is usually between about -2 and 16.
What does pKa tell you?
A lower pKa means a stronger acid that dissociates more fully in water, while a higher pKa means a weaker acid. Because pKa is a negative logarithm, each whole-number drop in pKa corresponds to a tenfold increase in Ka, so small pKa differences reflect large differences in acid strength.
What is Ka?
Ka is the acid dissociation constant, the equilibrium constant for an acid HA splitting into H+ and A- in water. It equals the product of the H+ and A- concentrations divided by the HA concentration. A large Ka means the equilibrium lies toward the dissociated ions, so the acid is strong.
How do I reverse pKa back to Ka?
Raise 10 to the power of the negative pKa: Ka = 10^(-pKa). For instance, a pKa of 4.74 gives Ka = 10^-4.74, which is about 1.8 x 10^-5. This is the exact inverse of the pKa formula.
What is the Ka to pKa formula?
pKa = -log10(Ka), where Ka is the acid dissociation constant and log10 is the base-10 logarithm. The negative sign turns the small fractional Ka into a positive number for most weak acids.
Official sources
- Acid dissociation constants and physical chemistry reference data: US National Institute of Standards and Technology (NIST). As at 25 June 2026.
Reviewed by the CalculatorHub team, edited by James Graham, 25 June 2026. See our methodology. This is general information, not financial, tax, legal or investment advice.