pOH Calculator
Find pOH from hydroxide concentration or pH.
Results update as you type.
How it works
pOH describes how basic a solution is by measuring its hydroxide ion concentration, and this calculator finds it from either the concentration or the pH. You enter the hydroxide ion concentration in moles per liter, or alternatively the pH, and it returns the pOH along with the corresponding pH and hydroxide concentration. The pOH is the negative base-ten logarithm of the hydroxide concentration, mirroring how pH is defined for hydrogen ions. A key relationship makes the two interchangeable: in water at 25 degrees Celsius, pH plus pOH always equals 14, because the product of hydrogen and hydroxide ion concentrations is a constant. So a solution with a pH of 9 has a pOH of 5, indicating it is basic. Low pOH means a high hydroxide concentration and a strongly basic solution, while high pOH means a low hydroxide concentration and an acidic solution. Chemists use pOH alongside pH when working with bases, buffer calculations, and titrations, since it can be more direct to reason about hydroxide ions for basic solutions. This calculator lets you move freely between hydroxide concentration, pOH, and pH, whichever you start from. The concentration must be positive for its logarithm to be defined. Enter a hydroxide concentration or a pH to compute the pOH and the related values.
pOH = −log₁₀[OH⁻] ; pOH = 14 − pH
pOH measures the hydroxide ion concentration of a solution as the negative base-10 logarithm of that concentration. Because pH and pOH always sum to 14 in water at 25°C, you can also find pOH by subtracting pH from 14. The calculator accepts either the hydroxide concentration or the pH and derives the rest.
Source:Acid-base chemistry
We review all our tools from time to time to ensure accuracy.
Worked example
How to use this calculator
- 1
Enter your values
Enter your values in the fields above.
- 2
Adjust the options
Adjust hydroxide concentration [oh⁻] and or ph if they apply — sensible defaults are already set.
- 3
Read your result
pOH updates instantly as you type — no button to press.
Frequently asked questions
What is pOH?
pOH is a measure of the hydroxide ion concentration in a solution, defined as the negative base-ten logarithm of that concentration. It is the hydroxide counterpart to pH. A low pOH indicates a basic solution rich in hydroxide ions, while a high pOH indicates an acidic solution with few hydroxide ions.
How are pH and pOH related?
In water at 25 degrees Celsius, pH plus pOH always equals 14. This comes from the ion product of water, where the hydrogen and hydroxide ion concentrations multiply to a fixed constant. Because of this, knowing either pH or pOH lets you calculate the other by subtracting from 14, which the calculator does automatically.
How do you calculate pOH from concentration?
Take the negative base-ten logarithm of the hydroxide ion concentration in moles per liter. For example, a hydroxide concentration of 0.001, or ten to the minus three, gives a pOH of 3. The calculator performs this logarithm for you and also reports the equivalent pH and, if you entered pH instead, the hydroxide concentration.
What does a low pOH mean?
A low pOH means a high concentration of hydroxide ions, which corresponds to a strongly basic, or alkaline, solution. Since pH and pOH sum to 14, a low pOH also means a high pH. For instance, a pOH of 1 corresponds to a pH of 13, indicating a very basic solution such as a concentrated hydroxide.
When is pOH more useful than pH?
pOH is often more convenient when working directly with bases and hydroxide concentrations, such as in titrations of bases, buffer calculations involving weak bases, and problems where the hydroxide concentration is the known quantity. Reasoning in pOH avoids extra conversion steps, though pH and pOH describe the same solution and can always be interchanged.