About the pH Calculator
This pH calculator works in two directions. Enter a known pH, pOH, hydrogen-ion concentration [H⁺] or hydroxide concentration [OH⁻] and it returns the other three instantly. Or choose a strong or weak acid or base, enter its molar concentration (and Ka or Kb for weak ones), and it works out the equilibrium [H⁺] and the resulting pH.
It is designed for chemistry students checking homework, lab technicians preparing solutions and anyone who needs a quick sanity check on acidity. Weak acids and bases are solved with the exact quadratic rather than the "x is small" shortcut, so results stay accurate even when a sizeable share of the acid ionizes, and water's own ionization is added back so very dilute solutions never read past neutral (pH 7).
All values assume an aqueous solution at 25 °C, where the ion product of water Kw = 1.0 × 10⁻¹⁴ and pH + pOH = 14. Concentrations are in mol/L (M). Activity effects in concentrated solutions are ignored, so results above about 1 M are approximate.
With the default inputs, the ph is 2.88. Change any value above to recalculate instantly.
How to use the ph calculator
- 1Choose what you already know: a pH, pOH, an ion concentration, or an acid/base and its concentration.
- 2Enter the value — concentrations in mol/L, using e-notation for small numbers (1e-5).
- 3For a weak acid or base, enter its Ka or Kb from a data table.
- 4For a strong acid or base, set how many H⁺ or OH⁻ ions each formula unit releases.
- 5Read the pH, pOH, [H⁺] and [OH⁻] and whether the solution is acidic, neutral or basic.
Formula and method
pH is the negative base-10 logarithm of the hydrogen-ion concentration in mol/L, and pOH is the same for hydroxide. In water at 25 °C the product [H⁺][OH⁻] equals Kw = 1.0 × 10⁻¹⁴, so pH + pOH = 14 and knowing any one quantity fixes the other three.
A strong acid or base dissociates completely, so [H⁺] (or [OH⁻]) equals the concentration times the number of ions released; the calculator also adds the contribution of water so very dilute solutions never drop below neutral. A weak acid HA only partly dissociates: setting Ka = x²/(C − x) and solving the quadratic gives x = [H⁺] exactly. Weak bases use the same equation with Kb to find [OH⁻].
- [H⁺]
- Hydrogen (hydronium) ion concentration, mol/L
- [OH⁻]
- Hydroxide ion concentration, mol/L
- Kw
- Ion product of water, 1.0 × 10⁻¹⁴ at 25 °C
- Ka, Kb
- Acid / base dissociation constant
- C
- Initial (analytical) concentration of the acid or base, mol/L
Worked examples
0.1 M acetic acid
Acetic acid has Ka = 1.8 × 10⁻⁵. Solving x² / (0.1 − x) = 1.8 × 10⁻⁵ gives [H⁺] ≈ 1.333 × 10⁻³ M, so pH ≈ 2.87. Only about 1.33% of the acid molecules ionize.
0.01 M hydrochloric acid
HCl dissociates completely, so [H⁺] = 0.01 M = 10⁻² M and pH = 2. The pOH is 14 − 2 = 12.
0.05 M calcium hydroxide
Each Ca(OH)₂ releases two hydroxide ions, so [OH⁻] = 0.10 M, pOH = 1 and pH = 14 − 1 = 13.
From a measured pH
A pH of 8.3 (typical of seawater) means [H⁺] = 10⁻⁸·³ ≈ 5.01 × 10⁻⁹ M and pOH = 14 − 8.3 = 5.7.
From a hydroxide concentration
[OH⁻] = 2.5 × 10⁻³ M gives pOH = −log(0.0025) ≈ 2.60, so pH = 14 − 2.60 ≈ 11.40 and [H⁺] = 10⁻¹⁴ ÷ 0.0025 = 4.0 × 10⁻¹² M.
Frequently asked questions
How do you calculate pH from concentration?+
For a strong acid, take the negative base-10 log of its molar concentration: 0.001 M HCl has pH = −log(0.001) = 3. For a weak acid you must first find [H⁺] from Ka, then take the negative log.
What is the relationship between pH and pOH?+
At 25 °C, pH + pOH = 14 because [H⁺][OH⁻] = 1.0 × 10⁻¹⁴. A solution with pH 4 therefore has pOH 10. At other temperatures Kw changes, so neutral water is not exactly pH 7.
Why is a weak acid pH higher than a strong acid at the same concentration?+
A weak acid only partly dissociates, so it releases fewer H⁺ ions. 0.1 M HCl has pH 1, while 0.1 M acetic acid has pH about 2.87 because only around 1.3% of it ionizes.
Can pH be negative or above 14?+
Yes. Very concentrated strong acids (above 1 M) can have negative pH and concentrated strong bases can exceed 14. In those solutions activity differs from concentration, so simple calculations are only approximate.
When can I use the √(Ka·C) shortcut?+
The approximation [H⁺] ≈ √(Ka·C) works when less than about 5% of the acid ionizes. This calculator always solves the full quadratic, so you do not need to check the 5% rule.
What is the pH of pure water?+
Pure water at 25 °C has [H⁺] = [OH⁻] = 1.0 × 10⁻⁷ M, giving pH 7. Rainwater exposed to air is usually around pH 5.6 because dissolved carbon dioxide forms carbonic acid.