pH Calculator
Calculate pH, pOH, [H⁺] and [OH⁻] from any one of them — or the pH of a weak acid from its concentration and Ka.
How to use
- 1Choose what you know: pH, pOH, [H⁺], [OH⁻] or a weak acid.
- 2Enter the value (concentrations in mol/L).
How it's calculated
pH = −log₁₀[H⁺]; pOH = −log₁₀[OH⁻]; pH + pOH = 14 at 25 °C. Weak acid: [H⁺] = (−Ka + √(Ka² + 4Ka·C)) ÷ 2.
Frequently asked questions
What pH is neutral?
7 at 25 °C. Below 7 is acidic; above 7 is basic (alkaline).
Why is the pH scale logarithmic?
Each unit is a factor of 10 in [H⁺] — pH 3 is ten times more acidic than pH 4.
Does temperature matter?
Yes — the ion product of water changes with temperature, so neutral pH is about 6.14 at 100 °C. These results assume 25 °C.