pH calculator
Choose the quantity you know, type its value and read the other three. Concentrations are in mol/L and accept scientific notation such as 2.5e-4. All results assume water at 25 °C, where the ion product Kw is 1e-14.
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How to use it
- Select which quantity you know: pH, pOH, [H⁺] or [OH⁻].
- Type its value. Use scientific notation for tiny concentrations.
- Read the other three values and whether the solution is acidic, neutral or basic.
How it works
With Kw = 1e-14 at 25 °C: pH = −log₁₀[H⁺], pOH = 14 − pH, [H⁺] = 10^(−pH) and [OH⁻] = 10^(−pOH). Below 7 the solution is acidic and above 7 it is basic.
Worked example
[H⁺] = 2.5e-4 mol/L gives pH = −log₁₀(2.5e-4) = 4 − 0.398 = 3.602 and pOH = 14 − 3.602 = 10.398, so [OH⁻] = 4.0e-11 mol/L. pH 3 gives [H⁺] = 0.001 mol/L and pOH 11.
Tips
- Each pH unit is a factor of ten in [H⁺]: pH 3 is ten times more acidic than pH 4.
- Report pH with as many decimals as you have significant digits in the concentration.
Frequently asked questions
What is the pH formula?
pH = −log₁₀[H⁺], where [H⁺] is the hydrogen ion concentration in mol/L. In the same way pOH = −log₁₀[OH⁻]. A solution with [H⁺] = 1e-3 mol/L has pH 3.
How are pH and pOH related?
At 25 °C, pH + pOH = 14, because the product [H⁺] × [OH⁻] equals Kw = 1e-14. The relation changes with temperature, because Kw does, so this tool applies only near 25 °C.
Can pH be below 0 or above 14?
Yes. Concentrated strong acids and bases can give values outside 0 to 14. The calculator accepts them, but for very concentrated solutions the real pH differs from the ideal formula because activities differ from concentrations.
How do I find pH of a weak acid?
First find the equilibrium [H⁺] from the acid's Ka and concentration, then enter that [H⁺] here. This tool converts between the four quantities and does not solve equilibrium problems.