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pH Calculator

Calculate pH, pOH, and ion concentrations from measured ions or an ideal strong or weak acid/base solution.

By ChemistryCalc•Updated October 28, 2026•References
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Results update as you type. Choose units beside a measurement; changing units converts the current value.

On this pageCalculatorHow to useFormula & methodWorked exampleConcentration and dissociation are different inputsWhere this model stopsFrequently asked questionsRelated calculatorsSources

01 Set up your calculation

  1. Choose the information you know rather than treating every acid or base as fully dissociated.
  2. Enter the ion or analytical solute concentration and, for a weak species, its dissociation constant.
  3. Use pKw appropriate to the temperature; the default 14 represents approximately 25 °C.

02 The relationship

pH = −log₁₀[H⁺]; pOH = pKw − pH. Strong acid: [H⁺] = (C + √(C² + 4Kw))/2. Weak acid: solve [H⁺] = Kw/[H⁺] + CKa/(Ka + [H⁺]); base modes use the analogous hydroxide balance.

For the assumptions behind these results, read the explanation below. You can also explore Buffer pH and Henderson–Hasselbalch.

WORKED EXAMPLE

Follow the numbers

A hydrogen-ion concentration of 0.001 mol/L gives pH 3. At pKw 14, pOH is 11 and hydroxide concentration is 10⁻¹¹ mol/L. For a 0.1 mol/L weak acid with Ka = 1.8 × 10⁻⁵, the estimate is about pH 2.87528.

03 Concentration and dissociation are different inputs

A measured hydrogen-ion concentration can be converted directly into the concentration-based pH estimate. An analytical acid concentration is instead the total amount of acid added per volume. A weak acid dissociates only partly, so those two concentrations are not interchangeable.

Strong modes assume complete dissociation of one proton or one hydroxide equivalent per solute molecule. Weak modes model one neutral monoprotic acid or one monobasic base in otherwise pure water. They include water’s contribution by solving charge balance, which avoids predicting an acidic pH for an extremely dilute base or a basic pH for an extremely dilute acid.

04 Where this model stops

The thermodynamic definition of pH uses hydrogen-ion activity. This calculator approximates activity by molar concentration, which becomes less reliable at high ionic strength. It does not model mixtures, salts, buffers, or multiple dissociation steps.

For a polyprotic acid such as phosphoric acid, entering one constant does not solve the full speciation problem. Use a model that includes all relevant equilibria. Negative pH and values above 14 are mathematically possible, but concentrated solutions particularly require attention to activities and measurement conventions.

Questions from the notebook

Is neutral pH always seven?

No. Neutrality means equal hydrogen and hydroxide concentrations; the neutral pH is pKw/2 and changes with temperature.

Can I calculate a buffer here?

Use the buffer pH or Henderson–Hasselbalch calculator for an appreciable conjugate acid-base pair.

Why does a very dilute strong acid approach pH seven?

Water contributes ions too. The charge-balance calculation includes that contribution instead of using pH = −log C alone.

References & reading

Our explanations are written for ChemistryCalc. Read how we build and check our tools. These calculations are educational estimates where the underlying model requires assumptions.