pH formula
pH is the negative base-10 logarithm of the hydrogen ion concentration: pH = −log10([H⁺]). The companion value pOH is 14 − pH at 25 °C. Concentration here is in mol/L, the same unit as the molarity calculator.
The minus sign exists purely for convenience. Hydrogen ion concentrations are tiny numbers with negative exponents, so flipping the sign turns an awkward −4 into the familiar pH 4 and puts most everyday solutions on a clean 0 to 14 scale.
How to use this calculator
Enter the hydrogen ion concentration in mol/L. You can use scientific notation such as 1e-7 for neutral water. The calculator returns pH and pOH. To prepare a solution at a known concentration, the dilution calculator helps you reach the target.
Concentration must be positive; zero and negative values have no logarithm and are rejected. Note that the 14 in the pOH step only holds at 25 °C, because the ion product of water changes with temperature — at 50 °C the neutral point sits nearer pH 6.6, not 7.
Worked example
An acid with [H⁺] = 0.0001 mol/L (1 × 10⁻⁴) has pH = −log10(0.0001) = 4 and pOH = 14 − 4 = 10. Lower pH means a stronger acid; pH 7 is neutral.
Because the exponent is a clean −4, the pH is a whole number; most real concentrations give a decimal instead. Values outside 0 to 14 are possible for very concentrated acids and bases, so a negative pH is unusual but not automatically an input error. For the hydroxide side of the same equilibrium, use the pH and pOH calculator.
Frequently asked questions
- What is pH?
- pH is a measure of acidity equal to the negative base-10 logarithm of the hydrogen ion concentration in mol/L.
- What is pOH?
- pOH measures hydroxide concentration; at 25 °C it equals 14 minus pH, so pH and pOH always sum to 14.
- Can I enter scientific notation?
- Yes — values like 1e-7 are accepted for very small concentrations such as neutral water.
- What pH is neutral?
- A pH of 7 is neutral at 25 °C; below 7 is acidic and above 7 is basic.
- Why is a pH of 4 a hundred times more acidic than a pH of 6?
- Because the scale is logarithmic: each whole pH unit is a tenfold change in hydrogen ion concentration. Two units apart therefore means 10 × 10 = 100 times more hydrogen ions, not twice as many.