pH Calculator
Convert between pH, pOH and hydrogen ion concentration.
Please note: Provided for learning and for checking your own working. Institutions differ in the conventions and rounding they apply, so confirm the method against your own course materials.
What the pH Calculator does
pH measures how acidic a solution is on a logarithmic scale, defined as the negative base-ten logarithm of the hydrogen ion concentration. pOH does the same for hydroxide ions, and the two always sum to pKw — 14.00 in water at 25 °C.
Formula
pH = −log₁₀[H⁺][H⁺] = 10^(−pH)pOH = pKw − pH[H⁺] × [OH⁻] = 10^(−pKw)
Inputs explained
| Input | Unit | Required | Notes |
|---|---|---|---|
| Start from | one of 4 options | Yes | — |
| Hydrogen ion concentration | mol/L | In some modes | Scientific notation works: 1e-3 is the same as 0.001. Accepts more than 0. Shown Start from is Hydrogen ion concentration [H⁺]. |
| pH | number | In some modes | Shown Start from is pH. |
| pOH | number | In some modes | Shown Start from is pOH. |
| Hydroxide concentration | mol/L | In some modes | Accepts more than 0. Shown Start from is Hydroxide concentration [OH⁻]. |
| pKw at this temperature | number | Optional | Water’s ion product. 14.00 at 25 °C, 13.26 at 50 °C. |
How to use it
- Choose Start from.
- Fill in the remaining inputs the form shows for your choice.
- Optionally add pKw at this temperature.
- Select Calculate.
Worked example
A solution with a hydrogen ion concentration of 0.001 mol/L.
- [H⁺]
- 0.001 mol/L
- pKw
- 14
pH = −log₁₀(0.001) = 3. pOH = 11, so [OH⁻] = 10⁻¹¹ mol/L. That is strongly acidic — 10,000 times more acidic than neutral water.
Frequently asked questions
Can pH go below 0 or above 14?
Yes. Concentrated acids can reach negative pH and strong bases exceed 14. The 0–14 range is just where most everyday solutions fall.
Why is neutral not always pH 7?
Neutral means equal hydrogen and hydroxide concentrations, which is pKw/2. That equals 7 only at 25 °C — at 100 °C neutral water sits at pH 6.14.