pH Calculator
Calculate pH from hydrogen ion concentration [H⁺], or find [H⁺] and [OH⁻] from a known pH value. Supports acids and bases.
pH
—
pOH —
[H⁺] mol/L —
[OH⁻] mol/L —
Classification —
Extended More scenarios, charts & detailed breakdown ▾
pH
—
pOH —
[OH⁻] mol/L —
Classification —
Professional Full parameters & maximum detail ▾
pH
—
pOH —
[H⁺] mol/L —
[OH⁻] mol/L —
Classification —
pKw at temperature —
[H⁺] activity (×10⁻⁷) —
Buffer Capacity (approx.) —
How to Use This Calculator
- Select whether you are converting from [H⁺] concentration to pH or from pH to concentration.
- Enter the known value.
- Results show pH, pOH, [H⁺], and [OH⁻] with solution classification.
Formula
pH = −log₁₀[H⁺]
[H⁺] = 10^(−pH)
pOH = 14 − pH | [OH⁻] = 10^(−pOH)
Example
Example: [H⁺] = 1×10⁻⁴ mol/L → pH = 4 (acidic) → pOH = 10, [OH⁻] = 10⁻¹⁰ mol/L.
Frequently Asked Questions
- pH is a logarithmic measure of hydrogen ion concentration in a solution: pH = −log₁₀[H⁺], where [H⁺] is in mol/L. The scale runs from 0 to 14 in aqueous solutions at 25°C. pH 7 is neutral (pure water); values below 7 are acidic (more H⁺ ions); values above 7 are basic or alkaline (fewer H⁺ ions). Because the scale is logarithmic, each unit represents a 10-fold change in concentration: pH 4 is 10× more acidic than pH 5, and 100× more acidic than pH 6. The pH scale was developed by Søren Sørensen in 1909.
- Pure water at 25°C has a pH of exactly 7.00. This is because water self-ionizes slightly: H₂O ⇌ H⁺ + OH⁻, and at 25°C, [H⁺] = [OH⁻] = 1.0 × 10⁻⁷ mol/L. Since pH = −log(1.0 × 10⁻⁷) = 7.00, water is neutral. At higher temperatures, the dissociation constant of water increases, so neutral pH is slightly below 7 at 37°C (body temperature), around 6.8. Distilled water exposed to air slowly absorbs CO₂ and becomes slightly acidic (pH 5.5–6), so fresh distilled water is a better reference for pH 7.
- pOH = −log₁₀[OH⁻], analogous to pH but measuring hydroxide ion concentration. In any aqueous solution at 25°C, pH + pOH = 14 (derived from the water dissociation constant Kw = 10⁻¹⁴). If pH = 9 (basic), pOH = 14 − 9 = 5, meaning [OH⁻] = 10⁻⁵ mol/L. If pH = 3 (acidic), pOH = 11, meaning [OH⁻] = 10⁻¹¹ mol/L. This calculator shows both pH and pOH, plus [H⁺] and [OH⁻] concentrations, for any input value.
- White vinegar (typically 5% acetic acid by volume) has a pH of approximately 2.4–3.0. The exact pH depends on the acetic acid concentration and dilution. Common food pH values for reference: lemon juice = 2.0–2.6, black coffee = 4.5–5.5, milk = 6.5–7.0, baking soda solution = 8.3, household bleach = 11–12.5. Battery acid (sulfuric acid) can reach pH 0–1. Blood is tightly regulated to pH 7.35–7.45 — deviations of 0.2 units can be medically serious.
- Use the formula: pH = −log₁₀([H⁺]). The negative log of the concentration gives a positive pH value for typical solution concentrations. For example: [H⁺] = 0.01 mol/L → pH = −log(0.01) = −(−2) = 2. [H⁺] = 1 × 10⁻⁴ mol/L → pH = −log(10⁻⁴) = 4. [H⁺] = 2.5 × 10⁻⁶ mol/L → pH = −log(2.5 × 10⁻⁶) = 5.60. Enter the [H⁺] value in this calculator and the pH is calculated instantly. For very dilute solutions below 10⁻⁷ mol/L, the self-ionization of water becomes significant and actual pH approaches but does not exceed 7.