What this calculator does
pH is the negative base-10 logarithm of the hydrogen ion concentration. A concentration of 1×10⁻⁷ mol/L gives a pH of 7, which is neutral at 25 °C because that is where hydrogen and hydroxide concentrations are equal.
Being logarithmic, each pH unit is a factor of ten in concentration. pH 4 is not slightly more acidic than pH 5; it has ten times the hydrogen ion concentration. Lemon juice near pH 2 is around a hundred thousand times more acidic than pure water, which the linear-looking scale conceals.
The formula
pH is minus the base-10 logarithm of the hydrogen ion concentration in mol/L, and the reverse is 10 to the power of minus pH. pOH is 14 minus pH, which holds at 25 °C because the ion product of water is 1×10⁻¹⁴ at that temperature. At other temperatures the neutral point is not 7.
| Term | Meaning |
|---|---|
| pH | Minus the log of hydrogen ion concentration. |
| pOH | The same for hydroxide. pH plus pOH is 14 at 25 °C. |
| Kw | The ion product of water, 1×10⁻¹⁴ at 25 °C, which sets the neutral point. |
| Neutral | Equal hydrogen and hydroxide concentrations, which is pH 7 only at 25 °C. |
The inputs explained
| Field | What to enter |
|---|---|
| Known value | Whether you are entering a concentration or a pH value. |
| Value | The known value. For concentration, enter in mol/L, using scientific notation such as 1e-7 for very small numbers. |
When to use it
Converting a measured concentration
Analytical results often come as concentrations and need expressing on the pH scale.
Working back to a concentration
A pH meter reading converts to the hydrogen ion concentration needed for equilibrium calculations.
Finding pOH
Base calculations often work in pOH, and the conversion is a subtraction from 14.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
What concentration does each pH represent?
A range of pH values with the corresponding concentrations.
| pH | [H⁺] | pOH | [OH⁻] |
|---|---|---|---|
| pH 1 | 0.1 mol/L | 13.00 | 1.0000e-13 mol/L |
| pH 4 | 0.0001 mol/L | 10.00 | 1.0000e-10 mol/L |
| pH 7 | 1.0000e-7 mol/L | 7.00 | 1.0000e-7 mol/L |
| pH 10 | 1.0000e-10 mol/L | 4.00 | 0.0001 mol/L |
| pH 13 | 1.0000e-13 mol/L | 1.00 | 0.1 mol/L |
Questions
Is pH 7 always neutral?
Only at 25 °C. Neutral means equal hydrogen and hydroxide concentrations, and the ion product of water changes with temperature. At 50 °C neutral water has a pH near 6.6, which is not acidic despite being below 7, because the hydroxide concentration has moved with it.
Can pH be negative or above 14?
Yes. The 0 to 14 range covers ordinary dilute aqueous solutions, not a hard limit. Concentrated strong acids can have a negative pH and concentrated bases can exceed 14, though the simple logarithmic relationship becomes unreliable at those concentrations because activity diverges from concentration.
How much more acidic is pH 4 than pH 5?
Ten times, because the scale is logarithmic. Each whole unit is a factor of ten in hydrogen ion concentration. Two units is a hundredfold, three is a thousandfold. This is why small-looking pH changes in a lake or a bloodstream can be serious.
What is the difference between pH and pOH?
pH tracks hydrogen ions and pOH tracks hydroxide ions, and at 25 °C they sum to 14. Either describes the solution completely, since knowing one fixes the other. Base chemistry is often more naturally expressed in pOH.
For the pH of a weak acid or base, see the weak acid and base pH calculator. For buffer pH, see the Henderson-Hasselbalch calculator.