pH Calculator
pH and pOH from a hydrogen or hydroxide ion concentration, a strong acid or base, or each other, with [H⁺] and [OH⁻], at 25 °C.
Result
pH: 2.9208
Uses concentrations in place of activities at 25 °C. For concentrated solutions, measure with a calibrated meter.
- pOH
- 11.0692
- Hydrogen ion concentration [H⁺], mol/L
- 1.2 × 10^-3
- Hydroxide ion concentration [OH⁻], mol/L
- 8.527 × 10^-12
- Acidic or basic
- Acidic at 25 °C: the pH is below 6.995, half of pKw.
https://calcgallery.com/chemistry/ph-calculator/
Results come from the published equations applied to the values you enter. They are not electrical design, conductor sizing, laboratory safety or medical guidance. Weather indices describe a standard person in standard conditions and are not a forecast. Where codes apply, local rules vary and must be checked. Read the full disclaimer
How to use the pH Calculator
- Choose what you know. A hydrogen or hydroxide ion concentration, a strong acid or base with its concentration, or a pH or pOH to convert.
- Enter the value. For a strong acid or base, also say how many H⁺ or OH⁻ ions each formula unit releases.
- Read the pH and pOH, both ion concentrations and whether the solution is acidic or basic.
The formulas
pH = −log₁₀[H⁺] [H⁺] = 10^−pHpOH = −log₁₀[OH⁻] [OH⁻] = 10^−pOHpH + pOH = pKw [H⁺] × [OH⁻] = Kwstrong acid: [H⁺] = (kC + √((kC)² + 4Kw)) ÷ 2strong base: [OH⁻] = (kC + √((kC)² + 4Kw)) ÷ 2C is the acid or base concentration in mol/L and k the number of H⁺ or OH⁻ each formula unit releases. Above about 10⁻⁵ mol/L the square root changes the pH by less than 0.0001 and [H⁺] is simply kC; below that, water’s own ions start to count.
Limits
The relations use concentrations where strictly they need activities, which is accurate for dilute solutions and increasingly approximate for concentrated ones. The page works at 25 °C only, because Kw changes with temperature.
What the diagram shows
The pH scale from 0 to 14, shaded on the acidic side of the neutral point, with a marker at your pH.
With your numbers
- [H⁺] = 1.2 × 10^-3 mol/L
- pH = −log₁₀[H⁺] = 2.9208
- pOH = pKw − pH = 13.99 − 2.9208 = 11.0692
Quick reference
Uses concentrations in place of activities at 25 °C. For concentrated solutions, measure with a calibrated meter.
| Example | Result |
|---|---|
| [H⁺] = 0.0012 mol/L | pH: 2.9208 |
| 0.01 M hydrochloric acid | pH: 2 |
| 0.0125 M potassium hydroxide | pH: 12.0869 |
| 0.01 M barium hydroxide | pH: 12.291 |
| 10⁻⁸ M hydrochloric acid | pH: 6.9735 |
| [OH⁻] = 0.001 mol/L | pH: 10.99 |
| pH 7.3 to [H⁺] | Hydrogen ion concentration [H⁺], mol/L: 5.012 × 10^-8 |
| pOH 4 to pH | pH: 9.99 |
Where the numbers come from
The fixed figures this calculator relies on, and the published source each one was checked against. How the calculators are built and checked
1 source
IAPWS R11-24, Revised Release on the Ionization Constant of H2O (2024), Table 3, 25 °C, the psat row (liquid density at 0.1 MPa below 100 °C): 13.99. https://iapws.org/technical-guidance/release/Ionization
Used for
- The ionization constant of water as pKw = −log10 Kw at 25 °C and 0.1 MPa. Chemistry courses round it to 14.00 (Kw = 1.0 × 10⁻¹⁴).
Common questions about the pH Calculator
- How is pH calculated?
- pH is the negative base-10 logarithm of the hydrogen ion concentration in moles per litre. Stomach acid at [H⁺] = 1.2 × 10⁻³ mol/L has a pH of −log(0.0012) = 2.92. Each step of one pH unit is a tenfold change in [H⁺].
- Why does pH + pOH come to 13.99 here and not 14?
- pH + pOH equals pKw, the negative logarithm of water’s ionisation constant. At 25 °C the International Association for the Properties of Water and Steam gives pKw = 13.99; courses round Kw to 1.0 × 10⁻¹⁴, which gives 14.00. Open the advanced options and set pKw to 14 to match a textbook answer; the pH from [H⁺] does not change either way, only the pOH and [OH⁻].
- Which acids and bases count as strong?
- Those that ionise completely in water. The common strong acids are hydrochloric, hydrobromic, hydroiodic, nitric, perchloric and sulfuric acid; the strong bases are the hydroxides of lithium, sodium and potassium and of calcium, strontium and barium. Barium hydroxide releases two OH⁻ per formula unit, so set the count to 2. Sulfuric acid releases its first H⁺ completely but its second only partly, so neither 1 nor 2 gives its exact pH.
- Why is 10⁻⁸ M hydrochloric acid not pH 8?
- Because an acid cannot make water basic. Pure water already holds about 10⁻⁷ mol/L of H⁺ from its own ionisation, ten times more than this acid adds. The page solves the charge balance [H⁺] = C + Kw ÷ [H⁺], which counts both sources, and gives 6.97, just on the acidic side of neutral.
- Does this work for weak acids like vinegar?
- No. A weak acid ionises only partly, and its pH depends on its acid dissociation constant, Ka, as well as its concentration. This page assumes complete ionisation. For a weak acid with its conjugate base, the buffer pH calculator applies.