pH Calculator
Strong or weak, acid or base: enter the concentration (and Ka or Kb for weak ones). Values assume 25 °C.
- Strong acid ionises fully: [H⁺] ≈ 0.01 M
- pH = −log[H⁺] = 2.00
Ask about this pH
AI answers can be wrong. The calculators on this page are deterministic; trust them for numbers.
The pH scale
Each pH unit is a factor of ten in [H⁺]. Lemon juice at pH 2 has a hundred thousand times more hydrogen ions than pure water at pH 7. Stomach acid is around 1–2, blood 7.35–7.45, household ammonia about 11.
Making a solution first? Find its concentration with the molarity calculator.
Questions students ask
How do you calculate pH?
pH = −log₁₀[H⁺], with [H⁺] in mol/L. A 0.01 M solution of a strong acid such as HCl has [H⁺] = 0.01 M and pH = 2.
How do I find pH from pOH?
At 25 °C, pH + pOH = 14 (because Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴). So pH = 14 − pOH.
How do I calculate the pH of a weak acid?
Use its Ka. For initial concentration C, solve x²/(C − x) = Ka for x = [H⁺]. The calculator solves the full quadratic, x = (−Ka + √(Ka² + 4KaC)) / 2, rather than the x ≪ C shortcut, so it stays accurate for fairly strong or very dilute acids.
Why is pure water pH 7?
Water self-ionises slightly: [H⁺] = [OH⁻] = 1.0 × 10⁻⁷ M at 25 °C, so pH = 7. At body temperature Kw is larger and neutral pH is about 6.8, though the water is still neutral.
Does this account for very dilute strong acids?
Yes. Below about 10⁻⁶ M, water’s own H⁺ matters, so the calculator solves [H⁺]² − C[H⁺] − Kw = 0. A 10⁻⁸ M HCl solution gives pH 6.98, not 8.