Chemistry

Instant, private, and free

pKa Calculator.

Calculate pKa from an acid dissociation constant Ka.

On-device calculationNo signup
01

Set your values

Results update as you type.

pKa: 5

pKa

5.000000

Educational formula calculator. Keep units consistent and verify assumptions against your laboratory or course specification.

Use Cases

Compare acid strengths

Convert Ka values to pKa to easily compare the relative strengths of different acids. Lower pKa means stronger acid.

Example: Acetic acid (Ka=1.8e-5) has pKa=4.74, while hydrochloric acid (Ka=1e6) has pKa=-6.

Check your chemistry homework

Verify your manual calculations when converting Ka to pKa in acid-base chemistry problems.

Example: If you calculated pKa=4.75 for a Ka of 1.78e-5, this calculator confirms it.

Frequently Asked Questions

What is pKa and how is it calculated?
pKa is the negative base-10 logarithm of the acid dissociation constant (Ka). It measures the strength of an acid in solution. The formula is pKa = -log10(Ka). Lower pKa values indicate stronger acids.
What does the Ka value represent?
Ka (acid dissociation constant) quantifies the extent to which an acid donates a proton (H+) to water. It is the equilibrium constant for the dissociation reaction. Larger Ka values mean the acid dissociates more, indicating a stronger acid.
Can I use this calculator for any acid?
Yes, as long as you have the Ka value for the acid. This calculator works for any acid where Ka is known, regardless of whether it's a weak or strong acid. Simply enter the Ka value and the pKa will be computed.

Tips & Common Mistakes

Tips

  • Ensure your Ka value is entered in standard decimal or scientific notation (e.g., 1.8e-5).
  • Remember that pKa is the negative logarithm, so a smaller pKa corresponds to a larger Ka and a stronger acid.
  • For polyprotic acids, use the Ka for each dissociation step separately to get the corresponding pKa values.
  • Double-check your Ka value for correct units; Ka is dimensionless, so no unit conversion is needed.

Common Mistakes to Avoid

  • Forgetting to take the negative logarithm: pKa = -log10(Ka), not log10(Ka).
  • Entering Ka as a percentage or with a unit, but Ka is dimensionless.
  • Confusing pKa with pH: pKa is a property of the acid, while pH is the acidity of a solution.

Last updated: August 13, 2026