pH Calculator
Convert between pH, pOH, hydrogen-ion and hydroxide-ion concentration — enter any one and get the other three.
From a concentration to pH, or from pH back to concentration.
How the ph calculator works
pH is the negative base-10 logarithm of the hydrogen-ion concentration, and at 25 °C pH + pOH = 14 (because K_w = 10⁻¹⁴). A 0.001 M strong acid has pH 3; pH 9 means [H⁺] = 10⁻⁹ M and [OH⁻] = 10⁻⁵ M. Choose what you know and the calculator fills in the rest, with a verdict on whether it is acidic, neutral or basic.
Formula: pH = −log₁₀[H⁺] · pOH = 14 − pH · [OH⁻] = 10^(−pOH)
Worked examples
| Inputs | pH | Note |
|---|---|---|
| [H⁺] = 0.001 M | 3 | pH 3 |
| pH 9 | 9 | [H⁺] = 1 × 10⁻⁹ |
| [OH⁻] = 0.01 M | 12 | pOH 2 → pH 12 |
| [H⁺] = 4.2e-5 M | 4.377 | pH 4.38 |
FAQFrequently asked questions
Why is pH 7 neutral?
Because in pure water at 25 °C [H⁺] = [OH⁻] = 10⁻⁷ M. At other temperatures K_w changes and neutral water is not exactly pH 7.
Is a strong acid's pH just −log of its concentration?
Yes for a strong monoprotic acid at ordinary concentrations, since it is fully dissociated. Weak acids dissociate partly and need K_a.
Can pH be negative or above 14?
Yes — a 2 M strong acid has pH about −0.3. The 0–14 range is conventional, not a law.
What is pOH?
−log₁₀[OH⁻]. At 25 °C pH + pOH = 14, so pOH 2 means pH 12.
Where these figures come from
- IUPAC — Standard atomic weights (2021 conventional values) — the molar-mass table
- NIST — CODATA 2018 fundamental physical constants — Avogadro constant, gas constant, speed of light
- NIST Chemistry WebBook — thermochemical data
- CSIRO — Australia's national science agency
Last checked: September 2026. Atomic masses are the IUPAC conventional values; constants are CODATA 2018; equations are the standard textbook forms.