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pH / pOH Converter

Convert any one of pH, pOH, [H⁺] or [OH⁻] to the others.

How it is calculated

You give one known quantity and say which kind it is: pH, pOH, [H⁺] or [OH⁻]. The tool turns it into pH first. From a concentration: pH = −log₁₀[H⁺], or pH = 14 + log₁₀[OH⁻]. From pOH: pH = 14 − pOH. Then it gives all four values: pOH = 14 − pH, [H⁺] = 10⁻ᵖᴴ and [OH⁻] = 10⁻ᵖᴼᴴ. Concentrations are in mol/L and must be above zero when they are the input; pH and pOH inputs may be any number, so values below 0 or above 14 are accepted without a warning. The 14 is valid at 25 °C, which is the only temperature the tool uses. pH and pOH are rounded to two decimals, and concentrations are written in scientific notation with two decimals.

Worked example

Known: [OH⁻] = 0.0002 mol/L, input type [OH⁻]. pOH = −log₁₀(0.0002) ≈ 3.70. pH = 14 − 3.70 = 10.30. Then [H⁺] = 10⁻¹⁰·³⁰ ≈ 5.00 × 10⁻¹¹ mol/L and [OH⁻] is returned as 2.00 × 10⁻⁴ mol/L. A pH above 7 means the solution is basic.

Try this example →

Common mistakes

  • Leaving the input type on pH while typing a concentration. A value such as 0.0002 would then be read as a pH of 0.0002, not as a concentration.
  • Choosing [H⁺] when the number you have is [OH⁻]. This gives a pH that is mirrored around 7 (3.70 instead of 10.30).
  • Entering the concentration in mmol/L or in percent. The tool expects mol/L.
  • Entering a pOH when you meant pH. The two add up to 14, so mixing them reverses acidic and basic.
  • Trusting a result outside the 0–14 range or at a temperature far from 25 °C. The tool accepts such values but the relationship between the two scales shifts with temperature.

About this calculator

This calculator converts between pH, pOH, hydrogen-ion concentration [H⁺], and hydroxide-ion concentration [OH⁻] at 25 °C, using pH = −log₁₀[H⁺], pOH = −log₁₀[OH⁻], and the relationship pH + pOH = 14 (since [H⁺][OH⁻] = Kw = 10⁻¹⁴ at that temperature). A solution is acidic when pH < 7, basic when pH > 7, and neutral at pH = 7.

Chemistry students and lab technicians use it to move quickly between whichever value they measured or were given and the others — for instance, converting a pH meter reading into ion concentration for a calculation, or checking pOH when working with basic solutions.

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Comments (1)

  • Editorial team

    A single pH step is a tenfold change in hydrogen ion concentration, so pH 3 is a hundred times more acidic than pH 5. As a check, enter 0.001 mol/L as the hydrogen concentration and the tool returns pH 3.00 and pOH 11.00. Everything here assumes 25 degrees Celsius, where pH plus pOH equals 14, so results for hot or cold solutions will differ. What temperature range would you like covered?

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