pH Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/5/2026
A solution's pH is calculated with pH = −log₁₀[H⁺], where [H⁺] is the hydrogen ion concentration in mol/L. For pure water ([H⁺] = 1 × 10⁻⁷ mol/L), the pH is exactly 7, the reference neutral value.
Explanation
pH (potential of hydrogen) measures how acidic or basic an aqueous solution is, on a scale generally running from 0 (very acidic) to 14 (very basic), with 7 as the neutral value at 25 °C. It's calculated from the hydrogen ion concentration [H⁺] with the formula pH = −log₁₀[H⁺]: the higher the H⁺ ion concentration, the lower the pH (an acidic solution). The scale is logarithmic, not linear: a solution with pH 4 is ten times more acidic (ten times more concentrated in H⁺) than a solution with pH 5, and a hundred times more acidic than a solution with pH 6. pOH, calculated here as pH + pOH = 14 (a relationship valid at 25 °C, since water's ionic product Kw = 10⁻¹⁴ at that temperature), symmetrically measures basicity: a low pOH corresponds to a strongly basic solution. Concentration is entered in scientific notation (a coefficient × 10 raised to a power), the most common way to express an H⁺ ion concentration, which is often a very small decimal value. To calculate a dilution or a molar concentration, see our dilution calculator and molar concentration calculator.
Example: [H⁺] = 5 × 10⁻³ mol/L
Inputs
Coefficient: 5. Exponent: −3 (i.e. [H⁺] = 0.005 mol/L).
Calculation
pH = −log₁₀(0.005) ≈ 2.30. pOH = 14 − 2.30 = 11.70.
Result
This solution has a pH of about 2.3: it's a strongly acidic solution.
Frequently asked questions
Why does the pH scale generally run from 0 to 14?
This range corresponds to water's ionic product (Kw = 10⁻¹⁴ at 25 °C), which relates the H⁺ and OH⁻ ion concentrations in water. In practice, pH can slightly exceed this range (negative values or values above 14) for solutions very concentrated in a strong acid or base — an edge case this calculator doesn't cover in detail.
Why does a pH difference of 1 correspond to a factor of 10?
Because pH uses a base-10 logarithmic scale: each pH unit represents an H⁺ ion concentration ten times larger or smaller. That's why acid rain at pH 4 is ten times more acidic than rain at pH 5, a gap that looks small on paper but represents a considerable difference in concentration.
Is the relationship pH + pOH = 14 always true?
No, it's only exact at 25 °C, the standard reference temperature in chemistry. Water's ionic product Kw varies with temperature, which slightly changes this relationship at other temperatures — a detail usually overlooked in a classroom setting or everyday use, but real in rigorous chemistry.