pH Calculator
Calculate pH, pOH and ion concentrations for acids, bases and weak acids.
About the pH Calculator
Choose a strong acid, strong base, weak acid or a concentration, and enter the concentration (and Ka for a weak acid). The calculator gives pH, pOH and the hydrogen and hydroxide ion concentrations at 25 °C.
pH is the negative base-10 logarithm of the hydrogen-ion concentration. At 25 °C water's ion product Kw = 1.0 × 10⁻¹⁴, so pH + pOH = 14. Strong acids dissociate completely; for a weak acid, [H⁺] comes from solving Ka = x² ÷ (C − x).
The formula
pH = −log₁₀[H⁺]; weak acid: [H⁺] = (−Ka + √(Ka² + 4 Ka C)) ÷ 2
Worked example
0.01 M hydrochloric acid (a strong acid)
- [H⁺] = 0.01 mol/L
- pH = −log₁₀(0.01) = 2
- pOH = 14 − 2 = 12
Things to keep in mind
- Assumes ideal dilute solutions (activity ≈ concentration) at 25 °C.
Questions people ask
Can pH be negative?
Yes, for very concentrated strong acids, though activity effects make it approximate.
Is pure water always pH 7?
Only at 25 °C; Kw changes with temperature.