pH Calculator
Enter any one of pH, pOH, hydrogen-ion concentration [H⁺] or hydroxide-ion concentration [OH⁻], and get the other three — plus whether the solution is acidic, neutral or basic (at 25 °C).
Quick answer: pH = −log₁₀[H⁺] and pH + pOH = 14 at 25 °C. So a solution with [H⁺] = 1×10⁻³ M has pH 3, pOH 11 and [OH⁻] = 1×10⁻¹¹ M — it's acidic (pH below 7).
pH measures how acidic or basic a solution is on a scale that runs from below 0 to above 14. It's the negative base-10 logarithm of the hydrogen-ion concentration. Because water self-ionises, the hydrogen and hydroxide concentrations are linked by the ion product Kw, so pH, pOH, [H⁺] and [OH⁻] are all convertible into one another.
The pH equations
pH = −log₁₀[H⁺] · [H⁺] = 10⁻ᵖᴴ
pOH = −log₁₀[OH⁻] · pH + pOH = 14 (at 25 °C)
[H⁺][OH⁻] = Kw = 1.0×10⁻¹⁴
pOH = −log₁₀[OH⁻] · pH + pOH = 14 (at 25 °C)
[H⁺][OH⁻] = Kw = 1.0×10⁻¹⁴
Worked example
A solution has [H⁺] = 1×10⁻³ M. pH = −log(1×10⁻³) = 3.00, so pOH = 14 − 3 = 11.00 and [OH⁻] = 10⁻¹¹ = 1×10⁻¹¹ M. A pH of 3 is acidic.
Frequently asked questions
How do I calculate pH from concentration?
Take the negative base-10 log of the hydrogen-ion concentration: pH = −log₁₀[H⁺]. For 0.001 M, pH = 3.
What is the relationship between pH and pOH?
At 25 °C they add to 14: pH + pOH = 14. So pOH = 14 − pH.
What pH is acidic, neutral or basic?
At 25 °C, pH 7 is neutral, below 7 is acidic, and above 7 is basic (alkaline).
Can pH be negative or above 14?
Yes. Very concentrated strong acids can have a pH below 0, and concentrated strong bases can exceed 14.
Related calculators
For general and educational use. Assumes 25 °C; Kw changes with temperature.