Choose what you know: a hydrogen ion concentration, a pH value, or the concentration of a strong acid or strong base. The calculator works out the rest at 25 degrees C.
You can type scientific notation such as 1e-7, or a plain decimal such as 0.0000001.
Assumes complete dissociation, which is a fair approximation for common strong acids such as HCl, HNO3, or HBr.
This calculation assumes complete dissociation and a temperature of 25 degrees C (Kw = 1e-14). It is a simplified estimate and does not replace a real measurement with a calibrated pH meter for applications such as pool water, laboratory work, or aquariums.
pH in everyday life and GCSE Chemistry
pH is one of the first quantitative scales UK students meet in Chemistry, from GCSE acids and alkalis through to A-level equilibrium calculations. It measures how acidic or alkaline a solution is on a scale that most substances fall between 0 and 14, using pH = -log10([H+]), where [H+] is the hydrogen ion concentration in mol/L. This tool lets you go either way: enter a concentration to get the pH, or enter a pH to find the concentration, and it also works out pOH and the hydroxide ion concentration [OH-] using Kw = [H+] x [OH-] = 1e-14 at 25 degrees C.
Everyday pH examples
| Substance | Typical pH |
|---|---|
| Battery acid | about 0 |
| Lemon juice | about 2 |
| Vinegar | about 2.5 |
| UK tap water | about 6.5 to 8.5 |
| Pure water | 7 (neutral) |
| Swimming pool water | 7.2 to 7.8 (target range) |
| Seawater | about 8.1 |
| Household bleach | about 12.5 |
| Oven cleaner | about 13.5 |
What this calculator cannot do
The simple relationship pH = -log10([H+]) assumes a dilute solution of a strong acid or strong base that dissociates completely, so the concentration entered equals the hydrogen or hydroxide ion concentration directly. Weak acids and bases, such as ethanoic acid or ammonia, only partially dissociate, and finding their pH needs an acid dissociation constant (Ka or Kb) and a quadratic equation this tool does not cover. Very concentrated or very dilute solutions can also push the real pH outside the neat 0-14 range, and temperature changes shift the value of Kw away from 1e-14. For pool water, aquariums, or laboratory work where accuracy matters, always confirm with a calibrated pH meter or test kit rather than relying on a calculation alone.
Worked example
A solution of hydrochloric acid (HCl) at 0.01 mol/L is a strong acid, so it dissociates completely and [H+] = 0.01 mol/L. This gives pH = -log10(0.01) = 2, a clearly acidic result. From there, pOH = 14 - 2 = 12, and [OH-] = 1e-14 / 0.01 = 1e-12 mol/L. Select "A strong acid concentration" above, enter 0.01, and check the same result. To convert a mass of acid into a molar concentration first, the Solution Concentration Calculator works out molarity from the mass of solute, its molar mass, and the solution volume.