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.

How It Works

Formula

pH = -\log_{10}[H_3O^+]

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

pH + pOH = 14

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

Variables, symbols and units

$pH$: negative base-10 logarithm of hydronium concentration
$pOH$: negative base-10 logarithm of hydroxide concentration
$[H_3O^+]$: hydronium concentration of the solution(mol/L)
$[OH^-]$: hydroxide concentration of the solution(mol/L)
$K_w$: water ion product used for the 25 C shortcut(10^-14 at 25 C)

Calculation method explained

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

What 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.

Why 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.

Does 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.

Can 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.

Can 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.