pOH calculator from OH- concentration
Enter [OH−] in mol/L, for example 4.2e-5, to get pOH and pH at 25 °C.
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How to use it
- Enter hydroxide concentration [oh−] (mol/l).
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
pOH = −log10[OH−]; pH = 14 − pOH; [H+] = 10^-14 ÷ [OH−].
Example
Inputs: Hydroxide concentration [OH−] (mol/L) = 4.2e-5. Result: pOH: 4.3768; pH: 9.6232; Hydrogen ion [H+]: 2.38095e-10 mol/L.
Limits and caveats
- Assumes 25 °C, where pH + pOH = 14.
- Treats concentration as activity, so accuracy drops above roughly 0.1 mol/L.
Questions about this tool
What is the pOH of 0.001 M NaOH?
NaOH is a strong base, so [OH−] = 0.001, pOH = 3 and pH = 11.
How are pH and pOH related?
They add up to 14 at 25 °C, so knowing one gives the other.
When is a solution basic?
When pOH is below 7, which means pH is above 7 at 25 °C.