pH Calculator
Calculator
\[ \text{pH} = -\log_{10}[\text{H}^+] \qquad \text{pH} + \text{pOH} = 14 \]
For weak acids/bases, Ka or Kb is used. Buffer pH uses the Henderson–Hasselbalch equation.
Strong Acid / Strong Base

Weak Acid
\[ [\text{H}^+] \approx \sqrt{K_a \cdot C} \]
Weak Base
\[ [\text{OH}^-] \approx \sqrt{K_b \cdot C} \]
Buffer Solution — Henderson–Hasselbalch
\[ \text{pH} = \text{p}K_a + \log\left(\frac{[\text{A}^-]}{[\text{HA}]}\right) \]
About This Tool

pH is defined as the negative base-10 logarithm of the hydrogen ion concentration, and is related to pOH by pH + pOH = 14 at 25 °C (based on the water autoionisation constant Kw = 1.0 × 10⁻¹⁴). This calculator covers four common acid-base scenarios. For strong acids and bases, which dissociate completely, [H⁺] or [OH⁻] equals the input concentration directly. For weak acids and bases, which only partially dissociate, the equilibrium concentration of H⁺ or OH⁻ is solved using the quadratic form of the dissociation expression — more accurate than the common approximation √(Ka·C). The buffer section uses the Henderson–Hasselbalch equation, pH = pKa + log([A⁻]/[HA]), to calculate the pH of a solution containing a weak acid and its conjugate base. All sections display pH, pOH, and the relevant ion concentrations as outputs.