pOH measures how basic a solution is, the same way pH measures how acidic it is — and the two are directly linked. This calculator converts between hydroxide-ion concentration, pOH, and pH so you can move freely between whichever value you're given.
How the pOH Calculator works
pOH is defined as pOH = −log₁₀[OH⁻], the negative base-10 logarithm of the hydroxide-ion concentration in mol/L. Because water self-ionizes into H⁺ and OH⁻ with an equilibrium constant Kw = 1.0 × 10⁻¹⁴ at 25 °C, pH and pOH always sum to 14 in an aqueous solution at that temperature. That means knowing any one of pH, pOH, [H⁺], or [OH⁻] is enough to derive the other three — which is exactly what each tab of this calculator does.
Inputs and what they mean
The pOH tab takes a hydroxide-ion concentration in mol/L — use scientific notation (1e-3) or a plain decimal (0.001), whichever is easier. The From pH tab takes a plain pH number, typically 0–14, and derives pOH via pH + pOH = 14. The Concentration tab either takes the molarity of a strong, fully-dissociating base like NaOH (assuming 1:1 dissociation) or lets you enter pOH directly to skip the molarity assumption entirely.
Limits and edge cases
The strong-base molarity mode assumes complete dissociation, which holds for strong bases like NaOH or KOH but not for weak bases (ammonia, amines) — those need a base dissociation constant (Kb) and an equilibrium calculation this tool doesn't perform. The pH + pOH = 14 relationship is specific to 25 °C; Kw rises with temperature, so the neutral point shifts slightly at other temperatures. Values outside the conventional 0–14 scale are mathematically valid for very concentrated solutions but fall outside where the scale is typically interpreted.