pOH is the negative base-10 logarithm of the hydroxide ion concentration: pOH = −log10[OH⁻]. At 25 °C, pH + pOH = 14. A 0.01 mol/L NaOH solution has [OH⁻] = 10⁻² mol/L, so its pOH is 2 and its pH is 12. Below 7 the solution is basic; above 7 it is acidic.
Chemistry students working through acid–base problem sets, and lab staff preparing buffers or titrant, often have one quantity and need the rest. This calculator starts from whichever you know — the hydroxide ion concentration, the hydrogen ion concentration, or the pH — and returns pOH to two decimals, the matching pH, both ion concentrations in scientific notation, and whether the solution is acidic, basic or neutral. The working is printed line by line and a scale places the pOH between 0 and 14. One limit: it uses pKw = 14.00, which holds at 25 °C only. At 37 °C pKw is about 13.6, so neutral water sits near pOH 6.8, and the tool does not correct for temperature or for ion activity in concentrated solutions.
Pick [OH⁻] concentration, [H⁺] concentration or pH from the three buttons.
Type the concentration in mol/L (decimals such as 0.0001 work) or the pH value.
pOH, pH, both concentrations and the acid/base label update as you type, with the steps beneath.
Result pOH: 2
pH: 12
Solution is: Basic
[OH⁻]: 1.00 × 10^-2 mol/L
[H⁺]: 1.00 × 10^-12 mol/L
NaOH dissociates fully, so [OH⁻] equals the base concentration. The log of 10⁻² gives a pOH of 2 and a pH of 12.
Result pOH: 11
pH: 3
Solution is: Acidic
[OH⁻]: 1.00 × 10^-11 mol/L
[H⁺]: 1.00 × 10^-3 mol/L
With pKw = 14 at 25 °C, a pH of 3 leaves a pOH of 11; the solution is acidic.
Result pOH: 10
pH: 4
Solution is: Acidic
[OH⁻]: 1.00 × 10^-10 mol/L
[H⁺]: 1.00 × 10^-4 mol/L
The pH is 4, so pOH is 14 − 4 = 10 and [OH⁻] is 1.00 × 10^-10 mol/L.
pOH = −log10[OH⁻]; pH + pOH = pKw = 14; [OH⁻] = 10^(−pOH)
[OH⁻][H⁺]pKwWater self-ionises so that [H⁺][OH⁻] = Kw = 1.0 × 10⁻¹⁴ at 25 °C. Taking the negative log of both sides gives pH + pOH = 14. From a hydroxide concentration the tool takes the log directly; from [H⁺] it finds pH first and subtracts from 14; from pH it subtracts directly. The concentrations are then recovered by raising 10 to the negative exponent.
pH tracks hydrogen ion concentration and pOH tracks hydroxide ion concentration. They move in opposite directions and, at 25 °C, always add up to 14, so a pH of 9 means a pOH of 5.
At 25 °C pure water has [OH⁻] = 1.0 × 10⁻⁷ mol/L, giving a pOH of exactly 7. At body temperature, 37 °C, pKw falls to roughly 13.6 and neutral water has a pOH near 6.8.
Yes. A hydroxide concentration above 1 mol/L, such as 2 mol/L NaOH, gives a pOH of about −0.30. At those strengths the ideal relation becomes approximate because ion activity is lower than concentration, so the tool adds a note.
Multiply the molar concentration by the number of hydroxide ions per formula unit. For 0.005 mol/L Ca(OH)₂, [OH⁻] is 0.010 mol/L, which gives a pOH of 2.00.
A weak base only partly reacts with water, so [OH⁻] is much lower than its formal concentration. Solve the Kb equilibrium first, for example [OH⁻] ≈ √(Kb × C), then enter that hydroxide value here.
For strong base solutions the hydroxide concentration is the number you know directly from the recipe, so pOH falls out in one logarithm. Many textbook titration and buffer problems on the basic side are solved in pOH first and converted to pH at the end.
Using this tool in an assignment, article or report? Copy a citation, or a link for your website.
Tools Directory Editorial Team. (2026, October 17). pOH Calculator. Tools Directory. Retrieved October 28, 2026, from https://tools.milunir.com/chemistry/solutions-and-concentration/poh-calculator/
Tools Directory Editorial Team. “pOH Calculator.” Tools Directory, 17 Oct. 2026, https://tools.milunir.com/chemistry/solutions-and-concentration/poh-calculator/. Accessed 28 Oct. 2026.
Tools Directory Editorial Team. “pOH Calculator.” Tools Directory. Last modified October 17, 2026. Accessed October 28, 2026. https://tools.milunir.com/chemistry/solutions-and-concentration/poh-calculator/.
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