pH Calculator
Convert between pH, pOH, hydrogen-ion concentration [H⁺] and hydroxide-ion concentration [OH⁻]. Set pKw for the conditions of your problem instead of assuming that it is always 14.
pH and pOH formulas
pH = −log₁₀[H⁺] | [H⁺] = 10−pH
pOH = −log₁₀[OH⁻] | [OH⁻] = 10−pOH
pH + pOH = pKw | Kw = [H⁺][OH⁻] = 10−pKw
Neutrality occurs when pH = pOH = pKw/2. At pKw = 14, neutral pH is 7. A pH below the neutral value is acidic; a pH above it is basic.
How to use this calculator
- Select whether you know pH, pOH, [H⁺] or [OH⁻].
- Enter the value. For a concentration, select mol/L, mmol/L, µmol/L or nmol/L.
- Enter pKw. Keep 14.000 when that is the assumption specified for your exercise.
- Select Calculate to obtain all four related values, Kw and the acidic/basic classification.
Checked examples
| Known value | pKw | Results |
|---|---|---|
| pH = 7 | 14 | pOH = 7; [H⁺] = [OH⁻] = 1×10⁻⁷ mol/L |
| pH = 3 | 14 | pOH = 11; [H⁺] = 1×10⁻³ mol/L |
| pOH = 4 | 14 | pH = 10; [OH⁻] = 1×10⁻⁴ mol/L |
| [H⁺] = 0.01 mol/L | 14 | pH = 2; pOH = 12 |
| [OH⁻] = 2.5 mmol/L | 14 | pOH ≈ 2.60206; pH ≈ 11.39794 |
| [H⁺] = 500 nmol/L | 14 | pH ≈ 6.30103; acidic |
| pH = −1 | 14 | [H⁺] = 10 mol/L; pOH = 15 |
| pH = 6.63 | 13.26 | pOH = 6.63; neutral under that pKw |
Scope and limits
- The equations use an ideal concentration approximation. Thermodynamic pH is defined through hydrogen-ion activity, so concentrated solutions require activity coefficients and may not match this estimate.
- The calculator does not solve weak-acid or weak-base equilibrium from Ka, Kb, buffer composition or titration data.
- pKw depends on temperature, pressure and solvent. Enter the value appropriate to the stated conditions.
- pH and pOH inputs are supported from −300 to 300; pKw must be greater than 0 and no more than 300.
- Concentration inputs must be positive and are converted internally to mol/L.