pOH Calculator
pOH is the negative base-10 logarithm of the molar hydroxide ion concentration in an aqueous solution. It is the hydroxide counterpart to pH and measures how basic a solution is. This calculator converts a hydroxide ion concentration to pOH, then uses the self-ionization constant of water to report the corresponding pH and the hydrogen ion concentration. At 25 C the constant pKw equals 14.00, so pH and pOH always sum to 14.00, but the calculator lets you change pKw to model other temperatures. Enter a concentration in moles per liter to begin.
pOH formula
pOH = -log10([OH-])
pH = pKw - pOH (pKw = 14.00 at 25 C)
[H+] = 10^(-pH)
Kw = [H+][OH-] = 10^(-pKw)
The hydroxide concentration must be greater than zero. The classification compares pOH to 7 at 25 C: pOH below 7 is basic, exactly 7 is neutral, and above 7 is acidic.
pOH context
- The self-ionization of water gives Kw = [H+][OH-], equal to 1.0 x 10^-14 at 25 C.
- Taking negative logarithms yields pH + pOH = pKw, which is 14.00 at 25 C.
- pOH is most useful when a strong base concentration is known directly, such as a sodium hydroxide solution.
- Kw rises with temperature, so pKw falls below 14 in warm water and neutral pH drops below 7.
- Strong bases dissociate fully, so [OH-] equals the base molarity times the number of hydroxide ions released per formula unit.
pOH: frequently asked questions
What is pOH?
pOH is the negative base-10 logarithm of the hydroxide ion concentration: pOH = -log10[OH-]. It measures the basicity of a solution. A low pOH means a high hydroxide concentration (more basic), while a high pOH means few hydroxide ions (more acidic).
How are pH and pOH related?
At 25 C, pH + pOH = 14.00. This comes from the ion product of water, Kw = [H+][OH-] = 1.0 x 10^-14 at 25 C. Taking the negative logarithm of both sides gives pH + pOH = pKw = 14.00. The relationship changes at other temperatures because Kw is temperature dependent.
How do I find pOH from hydroxide concentration?
Take the negative base-10 logarithm of the molar hydroxide concentration. For example, if [OH-] = 1.0 x 10^-3 mol/L, then pOH = -log10(0.001) = 3.00. The solution is basic because pOH is below 7.
Why does pH + pOH not always equal 14?
The value 14 only applies at 25 C, where the self-ionization constant of water pKw equals 14.00. At higher temperatures Kw increases and pKw falls below 14, so neutral pH is below 7 and pH + pOH equals the actual pKw at that temperature. This calculator assumes 25 C unless you change the pKw input.
What pOH is neutral?
At 25 C a neutral solution has pOH = 7.00 (equal to pH = 7.00), because [H+] = [OH-] = 1.0 x 10^-7 mol/L. Below pOH 7 the solution is basic; above pOH 7 it is acidic.
Official sources
- IUPAC Gold Book: pH definition and logarithmic acidity scales.
- NIST Chemistry WebBook: thermochemical and aqueous reference data.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.