pH & pOH Calculator

Enter any one of pH, pOH, the hydrogen-ion concentration or the hydroxide-ion concentration, and the other three follow. pH is −log₁₀[H⁺] with the concentration in moles per litre, pOH is the same for hydroxide, and the two are tied together by the ion product of water: at 25 °C, Kw = [H⁺][OH⁻] = 10⁻¹⁴, which is where the familiar pH + pOH = 14 comes from. Concentrations are shown in scientific notation, since the interesting values run from about 1 down to 10⁻¹⁴.

Because the scale is logarithmic, a single unit is a factor of ten. A solution at pH 4 is not slightly more acidic than one at pH 5, it has ten times the hydrogen-ion concentration; lemon juice near pH 2 is around a hundred thousand times more acidic than pure water. That compression is what makes the scale usable, and it is also why averaging pH values is a mistake — averaging the concentrations and taking the logarithm afterwards gives a different, correct answer.

The rule everyone remembers is the one that needs the most care: pH 7 is neutral only at 25 °C. Kw is temperature-dependent because water's self-ionisation is endothermic, so heating water makes more of both ions. At 100 °C, Kw is about 5 × 10⁻¹³ and neutral water sits at pH 6.14; at 0 °C it is about 1 × 10⁻¹⁵ and neutral is pH 7.47. In every case the water is still neutral — equal amounts of both ions — so the number moved while the chemistry did not. This calculator uses the 25 °C constant, which is what a chemistry problem set means unless it says otherwise.

Two more limits worth knowing. The scale does not stop at 0 and 14: concentrated hydrochloric acid has a negative pH and concentrated sodium hydroxide goes above 14, and nothing in the definition forbids either. And pH is formally defined in terms of ion activity rather than concentration, which are close enough to interchangeable in dilute solution and diverge as concentration rises — so a calculated pH for a strong, concentrated solution is an approximation, and a measured one from a calibrated electrode is the number to trust.