pH calculator
pH = −log₁₀[H⁺] · pOH = 14 − pH at 25 °C, and [OH⁻] = 10^(−pOH).
Values outside pH 0–14 are accepted with a warning and are never clamped.
Pick the value you have and enter it — pH, pOH, [H⁺], and [OH⁻] all appear here.
Formula and how to use it
pH is the negative base-10 logarithm of the hydrogen-ion concentration: pH = −log₁₀[H⁺], with [H⁺] in mol/L, and the reverse is [H⁺] = 10^(−pH). Because water self-ionises, pH + pOH = 14 at 25 °C, and [OH⁻] = 10^(−pOH) follows from the same logarithm. At 25 °C a neutral solution has pH 7, values below 7 are acidic, and values above 7 are basic.
Worked example: a solution with [H⁺] = 0.001 mol/L (1 × 10⁻³ M) has pH = 3.00, so pOH = 14 − 3 = 11 and [OH⁻] = 10⁻¹¹ mol/L. For a strong monoprotic acid such as HCl, which dissociates completely, the acid concentration equals [H⁺] directly; for a strong base such as NaOH the concentration gives [OH⁻], so pOH = −log₁₀[OH⁻] is computed first.
This calculator converts between pH, pOH, [H⁺], and [OH⁻] from any one known value, and estimates pH from a strong acid or strong base concentration. Weak acids, weak bases, and polyprotic species do not dissociate fully, so they need an equilibrium (Ka) treatment and are outside the strong-estimator's scope.