About this calculator
Enter a hydrogen ion concentration (mol/L) to get the pH, or enter a pH to get the concentration. You also get the pOH, the hydroxide concentration and whether the solution is acidic, neutral or alkaline.
You can type concentrations in E notation, such as 1e-4 for 0.0001.
Worked examples
Real numbers, worked out by the same calculator. Press “Use these numbers” to try one above.
[H⁺] = 0.0001 mol/L
- pH
- 4
- Hydrogen ion concentration [H⁺]
- 1 × 10^-4 mol/L
- pOH
- 10
- Hydroxide concentration [OH⁻]
- 1 × 10^-10 mol/L
- Nature (at 25 °C)
- acidic
A pH of 4 means [H⁺] = 1 × 10^-4 mol/L: the solution is acidic at 25 °C.
Show the working
- pH = −log₁₀[H⁺] = −log₁₀(1 × 10^-4) = 4.
- At 25 °C, pH + pOH = 14, so pOH = 14 − 4 = 10.
- [OH⁻] = 10^(−pOH) = 1 × 10^-10 mol/L.
- pH below 7 is acidic, 7 is neutral and above 7 is alkaline.
pH 3
- Hydrogen ion concentration [H⁺]
- 1 × 10^-3 mol/L
- pH
- 3
- pOH
- 11
- Hydroxide concentration [OH⁻]
- 1 × 10^-11 mol/L
- Nature (at 25 °C)
- acidic
A pH of 3 means [H⁺] = 1 × 10^-3 mol/L: the solution is acidic at 25 °C.
Show the working
- [H⁺] = 10^(−pH) = 10^(−3) = 1 × 10^-3 mol/L.
- At 25 °C, pH + pOH = 14, so pOH = 14 − 3 = 11.
- [OH⁻] = 10^(−pOH) = 1 × 10^-11 mol/L.
- pH below 7 is acidic, 7 is neutral and above 7 is alkaline.
pH 7.4 (blood)
- Hydrogen ion concentration [H⁺]
- 3.981 × 10^-8 mol/L
- pH
- 7.4
- pOH
- 6.6
- Hydroxide concentration [OH⁻]
- 2.512 × 10^-7 mol/L
- Nature (at 25 °C)
- alkaline (basic)
A pH of 7.4 means [H⁺] = 3.981 × 10^-8 mol/L: the solution is alkaline (basic) at 25 °C.
Show the working
- [H⁺] = 10^(−pH) = 10^(−7.4) = 3.981 × 10^-8 mol/L.
- At 25 °C, pH + pOH = 14, so pOH = 14 − 7.4 = 6.6.
- [OH⁻] = 10^(−pOH) = 2.512 × 10^-7 mol/L.
- pH below 7 is acidic, 7 is neutral and above 7 is alkaline.
What pH is
pH measures how acidic a solution is: pH = −log₁₀[H⁺], where [H⁺] is the hydrogen ion concentration in moles per litre. Pure water at 25 °C has [H⁺] = 10⁻⁷, so its pH is 7, which is neutral. Below 7 is acidic; above 7 is alkaline.
pOH and hydroxide
The hydroxide concentration works the same way: pOH = −log₁₀[OH⁻]. At 25 °C, pH + pOH = 14, so a pH of 3 means a pOH of 11 and [OH⁻] = 10⁻¹¹ mol/L.
A logarithmic scale
Each step of one pH unit is a tenfold change in concentration. A solution of pH 3 is ten times more acidic than pH 4 and a hundred times more acidic than pH 5.
Frequently asked questions
How do I calculate pH?
Take the negative base-10 logarithm of the hydrogen ion concentration in mol/L.
What pH is neutral?
pH 7 at 25 °C.
What is the pH of 0.001 mol/L hydrochloric acid?
A strong acid fully releases its hydrogen ions, so [H⁺] = 0.001 and the pH is 3.
Does temperature matter?
Yes: the neutral point and the pH + pOH = 14 rule are for 25 °C. This calculator uses 25 °C.
Formulas tested against hand-worked answers. Last reviewed 29 September 2026. These calculators do arithmetic only; they are not financial, tax or legal advice.