This pH calculator converts hydrogen ion concentration to pH and pOH, and works backwards from pH to concentration.
Chemistry
pH Calculator
Calculate pH and pOH from ion concentration or weak acid/base constants.
Calculator
About this calculator
Formula notes
pH = −log₁₀[H⁺]; pOH = 14 − pH at 25 °C; [H⁺] = 10^(−pH) and [OH⁻] = 10^(−pOH).
Worked examples
- Example: [H⁺] = 0.0001 mol/L gives pH = 4 and pOH = 10, matching the page defaults.
- A hydrogen-ion concentration of 2.5 x 10^-4 mol/L gives pH = -log10(2.5 x 10^-4), or approximately 3.60.
How to use it
- Choose what you know: [H⁺], [OH⁻], pH, or pOH.
- Enter the value with correct units.
- Read all four related values.
Frequently asked questions
Why is pH 4 acidic?
pH below 7 means [H⁺] exceeds 10⁻⁷ mol/L, so the solution is acidic.
Does pH + pOH always equal 14?
Only at 25 °C. At other temperatures the ion product of water shifts.
Limits and interpretation
- Concentration-based pH is an approximation because thermodynamic pH depends on ion activity, especially in concentrated solutions.
- Weak-acid and weak-base calculations require appropriate equilibrium assumptions and may need a full charge-balance solution.