pH and pOH Calculator

How it is calculated

Enter the hydrogen-ion concentration [H⁺] in mol/L (molar). The tool calculates pH = −log₁₀[H⁺] and then pOH = 14 − pH, rounding both to two decimals. The value 14 holds at 25 °C, where the product [H⁺] × [OH⁻] equals 10⁻¹⁴; at other temperatures it is somewhat different, so the pOH is only exact at 25 °C. The concentration must be greater than zero. Very large values are allowed: a concentration above 1 mol/L gives a negative pH (for example 2 mol/L gives −0.30), and the pOH then exceeds 14. The tool takes the concentration you give as the real [H⁺]. It does not work out [H⁺] from an acid's strength, so for weak acids you must find the equilibrium concentration first.

Common mistakes

• Typing the pH itself in the concentration field. The field expects the concentration in mol/L, such as 0.001, not 3.
• Entering the acid concentration of a weak acid such as acetic acid instead of its [H⁺]. Only a small part ionises, so the real pH is higher than the result.
• Applying the formula to extremely dilute acids. At 10⁻⁹ mol/L the tool gives pH 9.00 (a base), but water itself supplies about 10⁻⁷ mol/L of H⁺ and the true pH is just under 7.
• Using the pH + pOH = 14 rule at a temperature far from 25 °C without adjusting.
• Forgetting that pH is logarithmic: a change of 1 unit is a tenfold change in [H⁺], and 2 units is a hundredfold change.