pH Calculator

pH is a logarithmic scale used to measure the acidity or basicity of an aqueous solution. It is defined as the negative base-10 logarithm of the hydrogen ion concentration: pH = −log₁₀[H⁺]. The scale runs from 0 to 14 at standard conditions (25°C), where a pH of 7 is neutral (pure water), values below 7 are acidic, and values above 7 are basic (alkaline). Because the scale is logarithmic, each unit change in pH represents a 10-fold change in hydrogen ion concentration — a solution at pH 3 has 10 times more H⁺ ions than a solution at pH 4. The complementary measure pOH describes hydroxide ion concentration: pOH = −log₁₀[OH⁻]. At 25°C, pH + pOH = 14, which follows from the water dissociation constant Kw = 1.0 × 10⁻¹⁴.

1. Select a Calculation Mode: Choose from seven modes — convert [H⁺] to pH, pH to [H⁺], [OH⁻] to pOH, pOH to [OH⁻], pH ↔ pOH conversion, Ka + concentration to pH (weak acid), or Kb + concentration to pOH (weak base). Each mode shows the relevant input fields automatically.
2. Enter Your Values: Type your concentration, pH, pOH, Ka, or Kb value into the input field. For concentration inputs, you can use scientific notation (e.g. 1e-7 for 10⁻⁷ mol/L). The pH calculator accepts any valid positive number for concentrations and any value between 0 and 14 for pH/pOH inputs.
3. Review the Full Result: The pH calculator displays pH, pOH, [H⁺], and [OH⁻] simultaneously, along with a color-coded classification (Strongly Acidic through Very Strongly Basic), a visual pH scale bar showing where your result falls, and a list of nearby common substances for context.

• pH from [H⁺]: Applies pH = −log₁₀[H⁺] to convert hydrogen ion molar concentration to pH, then derives pOH = 14 − pH and [OH⁻] = 10^(−pOH).
• pH from Ka (weak acid): Uses the approximation [H⁺] ≈ √(Ka × C), valid when Ka ≪ C (less than 5% dissociation). This gives pH ≈ ½(pKa − log C). For strong acids, use the [H⁺] → pH mode directly.
• pOH from Kb (weak base): Uses the analogous approximation [OH⁻] ≈ √(Kb × C) to compute pOH, then derives pH = 14 − pOH. Valid for weak bases where Kb ≪ C.
• pH ↔ pOH Conversion: Applies the water equilibrium relationship pH + pOH = 14 (at 25°C) to convert between the two scales and compute both ion concentrations simultaneously.

All calculations in this pH calculator assume 25°C (298 K), where Kw = 1.0 × 10⁻¹⁴ and pH + pOH = 14. At other temperatures, Kw changes and the neutral point shifts — for example, at 37°C (body temperature), Kw ≈ 2.4 × 10⁻¹⁴ and neutral pH ≈ 6.8. The weak acid/base modesuse the standard approximation [H⁺] ≈ √(Ka × C), which is accurate when the degree of dissociation is less than 5%. For very weak acids (Ka < 10⁻¹⁰) or very dilute solutions, the approximation may introduce small errors. For strong acids and bases (HCl, NaOH, etc.), use the direct concentration modes since they dissociate completely.