pH Calculator

pH, pOH and ion concentrations for acids, bases and buffers.

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pH Calculator

pH, pOH, ion concentrations and buffers at 25 °C

Solute

Solution

Concentration (mol/L)

Use scientific notation such as 1e-5 if you like.

H+ or OH− released per formula unit

1 for HCl or NaOH, 2 for H2SO4 or Ca(OH)2.

pH2Strongly acidic

pOH12

[H+]0.01 M

[OH−]1.000e-12 M

How it works

pH measures how acidic a solution is: pH = −log10 of the hydrogen ion concentration. pOH does the same for hydroxide, and pH + pOH = 14 at 25 °C. Strong acids and bases ionise completely; weak ones only partly, set by Ka or Kb. A buffer mixes a weak acid with its conjugate base to resist pH change.

[H+] = concentration × 1 = 0.01 M, since a strong acid or base ionises fully.
pH = −log10[H+] = 2; pOH = 14 − pH = 12.

Each pH unit is a tenfold change in [H+].
A buffer works best within one pH unit of its pKa.
pKa = −log10 Ka. A bigger Ka means a stronger acid.
For H2SO4 or Ca(OH)2 set 2 in the release box; the second ionisation of H2SO4 is not complete in strong solution, so treat it as an estimate.

Standard acid–base equilibrium at 25 °C (Kw = 1.0 × 10⁻¹⁴). Strong: exact solution including water autoionisation. Weak: exact quadratic for HA ⇌ H+ + A−. Buffer: Henderson–Hasselbalch, pH = pKa + log10([A−]/[HA]).

Preparing tool

How to use pH Calculator

Pick Strong, Weak, Buffer or From pH at the top.
Enter the concentration and, for weak acids and bases, Ka or Kb.
Read pH, pOH and the ion concentrations, with the working below.

Read the detailed guide →

What are the limits of pH Calculator?

Values are for 25 °C (Kw = 1.0 × 10⁻¹⁴) and ideal solutions; activity effects at high concentration are ignored. Weak acids and bases ignore water's own ions, so avoid concentrations near 10⁻⁷ M. Nothing you enter leaves your device.