Weak Acid pH Calculator

Solved exactly, without the x ≪ C approximation.

Written and maintained by Mohit PatelLast checked August 4, 2026How we build these

1.8 × 10⁻⁵ for acetic acid. Enter as a decimal.

Concentration

2.875

1.333% ionised · pKa 4.745

pH2.8753

[H⁺]0.00133 M

pKa4.7447

Percent ionised1.3327%

This solves the quadratic x² + Ka·x − Ka·C = 0 exactly rather than assuming the ionisation is negligible against the starting concentration. That approximation is taught because it is convenient, and it fails quietly for dilute solutions or acids that are not very weak — by around 5% ionisation it is already introducing visible error, and this page avoids the question by not making the assumption.

How the Weak Acid pH Calculator works

This solves the quadratic exactly rather than assuming ionisation is negligible against the starting concentration. That approximation is taught because it is convenient, and it fails quietly for dilute solutions or acids that are not very weak.

Also known as: ka to ph calculator · percent ionization calculator · ph of a weak acid · acid dissociation calculator

Where to go next

The Weak Acid pH question rarely arrives on its own. These are the ones that usually come with it:

Henderson-Hasselbalch Calculator — Buffer pH, and where the buffer stops buffering.
pH Calculator — pH, pOH and both ion concentrations, on a log scale.
Titration Calculator — Equivalence by equivalents, not by moles.
ICE Table Calculator — Solved properly, with no assumption that x is small.

Sources

Frequently asked questions

How do I find the pH of a weak acid?

Solve x² + Ka·x − Ka·C = 0 for the hydrogen ion concentration, then take the negative logarithm. For 0.1 M acetic acid with Ka 1.8 × 10⁻⁵ the pH is 2.87.

When does the usual approximation fail?

Once ionisation exceeds about 5% of the starting concentration, which happens for dilute solutions and for stronger weak acids. This page avoids the question by not making the assumption.

What is percent ionisation?

The fraction of acid molecules that have donated their proton, as a percentage. It rises as the solution is diluted, which surprises people expecting concentration and ionisation to move together.

Put this calculator on your own site

Free to use, on any site, commercial or not. Paste this where you want it to appear. It is a plain iframe, so it works in WordPress, Squarespace, Wix, Webflow, Ghost and anything else that accepts HTML.

The one-line version

<iframe src="https://www.thecalclibrary.com/embed/weak-acid-ph-calculator" width="100%" height="640" style="border:1px solid #e2e8f0;border-radius:12px" loading="lazy" title="Weak Acid pH Calculator"></iframe>
<p style="font:13px/1.5 system-ui,sans-serif;margin:6px 0 0;color:#64748b">Powered by <a href="https://www.thecalclibrary.com/weak-acid-ph-calculator" style="color:#64748b">Weak Acid pH Calculator</a> from The Calc Library</p>

The only condition is that the credit line below the frame stays in place. That one line is what pays for the tool being free — it is how anyone else finds it.