Find pH, pOH and ion concentrations for a solution.
Calculated locally in your browser.
How do you calculate pH?
pH = −log₁₀[H⁺], and [H⁺] = 10^(−pH). At 25 °C, pH + pOH = 14 and [OH⁻] = 10^(−pOH). The scale is logarithmic, so each whole step is a tenfold change in hydrogen ion concentration. For example, a concentration of 1 × 10⁻³ mol/L gives pH = −log₁₀(0.001) = 3, which is acidic.
Understanding your result
The pH scale is logarithmic, so each whole step represents a tenfold change in hydrogen ion concentration — pH 3 is ten times more acidic than pH 4. The pH + pOH = 14 relationship holds at 25 °C; at other temperatures the constant changes slightly.
Formula and method
pH = −log₁₀[H⁺] and [H⁺] = 10^(−pH). At 25 °C, pH + pOH = 14, and [OH⁻] = 10^(−pOH).
Assumptions and limitations
This tool applies the ideal relationships pH equals minus log of hydrogen ion concentration and, at 25 degrees Celsius, pH plus pOH equals 14. It assumes activity equals concentration, which is only accurate in dilute solutions, and the 14 constant shifts at other temperatures. It does not model buffers, ionic strength or the behaviour of weak acids and bases.
Worked example
A hydrogen ion concentration of 1 × 10⁻³ mol/L gives pH = −log₁₀(0.001) = 3, which is acidic.
How to use this tool
- Choose whether you know the concentration or the pH.
- Enter the value.
- Read the pH, pOH and ion concentrations.
Common mistakes to avoid
- Forgetting that the scale is logarithmic, not linear.
- Entering a concentration of zero or a negative value.
- Assuming pH + pOH = 14 at every temperature.
About the pH Calculator
The pH Calculator converts between hydrogen ion concentration and pH, and also reports pOH and the hydroxide concentration. It tells you at a glance whether a solution is acidic, neutral or basic.
Who should use this tool
Chemistry students, teachers, lab technicians and hobbyists.
Benefits
- pH from concentration, or concentration from pH.
- Shows pOH and [OH⁻] as well.
- Classifies the solution as acidic, neutral or basic.
- Private — calculated entirely in your browser.
Practical use cases
- Finding the pH of an acid or base from its concentration.
- Converting a measured pH back to ion concentration.
- Checking chemistry homework.
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Frequently asked questions
What is the formula for pH?
pH is the negative base-10 logarithm of the hydrogen ion concentration: pH = −log₁₀[H⁺].
What does pH 7 mean?
A pH of 7 is neutral at 25 °C, where hydrogen and hydroxide ion concentrations are equal. Below 7 is acidic, above 7 is basic.
How are pH and pOH related?
At 25 °C they always add up to 14, so pOH = 14 − pH.