pOH Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/8/2026
At 25°C, pOH is calculated with pOH = 14 − pH. A solution with pH 9 therefore has a pOH of 5, corresponding to a hydroxide ion concentration of 10⁻⁵ mol/L.
Explanation
pOH measures the basicity of a solution, the same way pH measures its acidity: the lower the pOH, the more basic the solution. At 25°C, pH and pOH are linked by a simple relationship that follows from water's ion product (Kw = 10⁻¹⁴): their sum always equals 14 (pH + pOH = 14). So a neutral solution (pH = 7) also has a pOH of 7; an acidic solution (pH < 7) has a pOH above 7; a basic solution (pH > 7) has a pOH below 7. Once pOH is known, the hydroxide ion concentration [OH⁻] is calculated directly with [OH⁻] = 10^(−pOH), the same logarithmic principle as the relationship between pH and H⁺ ion concentration (see our pH calculator). This pH + pOH = 14 relationship only holds at 25°C: water's ion product varies slightly with temperature, which shifts this sum at other temperatures.
Example: a solution with pH 9
Inputs
pH: 9.
Calculation
pOH = 14 − 9 = 5. Hydroxide ion concentration = 10⁻⁵ = 0.00001 mol/L.
Result
This solution has a pOH of 5 and a hydroxide ion concentration of 10⁻⁵ mol/L.
Frequently asked questions
Why is pH + pOH exactly 14?
Because the product of H⁺ and OH⁻ ion concentrations in water stays constant at a given temperature (Kw = [H⁺] × [OH⁻] = 10⁻¹⁴ at 25°C). Taking the negative logarithm of this relationship directly gives pH + pOH = 14, a mathematical consequence of water's ion product, not a coincidence.
Does this relationship hold at any temperature?
No, it is only exact at 25°C. Water's ion product (Kw) varies with temperature: it increases as temperature rises, which shifts the pH + pOH sum below 14 at high temperature, and above 14 at low temperature.
How should a very low hydroxide ion concentration like 10⁻¹⁰ mol/L be interpreted?
Such a low concentration corresponds to a clearly acidic solution (high pOH, so low pH): hydroxide ions are present in a tiny amount compared with H⁺ ions, which dominate the solution. The logarithmic scale of pH and pOH allows these extremely low or high concentrations to be expressed compactly, without directly handling powers of 10.