CalcLibrary

pH and pOH Calculator

Convert pH, pOH, [H3O+], and [OH-] for one dilute aqueous solution at 25 C. Enter one known value and get the full matching acid-base set without hand-running the logarithms.

Examples

Entering 1 mmol/L for [H3O+] gives pH 3 and keeps the solution clearly acidic.

pH
3
pOH
11
[H3O+] concentration
0.001 mol/L
[OH-] concentration
1 × 10^-11 mol/L
Solution classification
Acidic

Assumes dilute aqueous solutions at 25 C only. This page does not model titrations, weak-acid or weak-base equilibrium, buffers, activity corrections, temperature compensation, water-quality compliance, or medical interpretation.

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Examples

How It Works

Formula

pH=−log⁡10[H3O+]pH = -\log_{10}[H_3O^+]

pOH=−log⁡10[OH−]pOH = -\log_{10}[OH^-]

pH+pOH=14pH + pOH = 14

[H3O+][OH−]=10−14[H_3O^+][OH^-] = 10^{-14}

Variables

pHpH

negative base-10 logarithm of hydronium concentration

pOHpOH

negative base-10 logarithm of hydroxide concentration

[H3O+][H_3O^+]

hydronium concentration of the solution(mol/L)

[OH−][OH^-]

hydroxide concentration of the solution(mol/L)

KwK_w

water ion product used for the 25 C shortcut(10^-14 at 25 C)

Pick the acid-base quantity you already know, enter one value, and the calculator derives the other three values for the same solution. The result panel reports pH, pOH, [H3O+], [OH-], and a plain-language classification so you can read the chemistry without doing the logarithms by hand.

This calculator assumes dilute aqueous solutions at 25 C.

  • If you start from pH, it uses [H3O+] = 10^-pH, then pOH = 14 - pH, and [OH-] = 1e-14 / [H3O+].
  • If you start from pOH, it uses [OH-] = 10^-pOH, then pH = 14 - pOH, and [H3O+] = 1e-14 / [OH-].
  • If you start from a concentration entry, the page first converts mmol/L or umol/L into mol/L before applying the same logarithmic relationships.

The result is a fast worksheet or lab-bench conversion for one already-defined solution, not a broader equilibrium solver.

Frequently Asked Questions

01What relationships does this calculator use?
It uses pH = -log10[H3O+], pOH = -log10[OH-], pH + pOH = 14, and [H3O+][OH-] = 1e-14. Those are the standard dilute-aqueous shortcuts for 25 C.
02Why does the page keep saying 25 C?
Because the shortcut pH + pOH = 14 and the water ion product 1e-14 are temperature-dependent. This page intentionally fixes the scope to dilute aqueous solutions at 25 C instead of pretending those constants stay the same everywhere.
03Does this handle buffers, weak acids, weak bases, or titrations?
No. It converts one already-known acid-base measure into the other three for the same solution. It does not solve weak-acid equilibrium, buffer behavior, titration curves, or concentration changes during mixing.
04Can pH or pOH fall outside 0 to 14 here?
Yes, the logarithmic relationships can produce values outside 0 to 14. This page still remains an ideal dilute-aqueous conversion tool, not a full real-solution chemistry model.
05Can I use this for water-quality compliance or health interpretation?
No. It is a conversion helper, not a regulatory water-testing tool and not a medical interpretation tool.

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