About the pH Calculator
The pH scale tells you how acidic or basic a solution is. This pH calculator converts between pH, pOH, the hydrogen ion concentration [H⁺] and the hydroxide ion concentration [OH⁻], and also works out the pH of a solution from the concentration of a strong acid or strong base, or of a weak acid or weak base using its Ka or Kb. For a weak acid or base it also shows what percentage of the molecules ionise. It assumes water at 25 °C, where pH and pOH add up to 14, and corrects for the water's own ions in very dilute solutions. It is a handy check for chemistry homework and lab preparation.
Updated October 2026
How to use the pH Calculator
- 1Choose what you know: pH, pOH, an ion concentration, or the concentration of an acid or base.
- 2Enter the value. Concentrations are in moles per litre and can be written like 1e-3.
- 3For a weak acid or base, enter Ka or Kb as well.
- 4For acids or bases that release more than one ion, choose the number.
- 5Read the pH, pOH, [H⁺], [OH⁻] and whether the solution is acidic, neutral or basic.
pH values at a glance
- Battery acid: about 0 to 1; stomach acid: about 1.5 to 3.5
- Lemon juice: about 2; vinegar: about 2.5; coffee: about 5
- Pure water: 7 (neutral)
- Baking soda solution: about 8.3; sea water: about 8.1
- Household ammonia: about 11 to 12; bleach: about 12.5
Strong or weak?
Strong acids and bases, such as HCl, HNO₃, NaOH and KOH, split completely into ions in water. Weak ones, like acetic acid and ammonia, only ionise partly, and the smaller their Ka or Kb the less they do. A weak acid at the same concentration as a strong acid gives a much higher pH. If you aren't sure which to use, look up the acid in a table of Ka values.
Related tools
The Molarity Calculator prepares solutions of a given concentration, the Molecular Weight Calculator gives the molar mass, the Log Calculator and Scientific Notation Converter help with exponents and the Exponent Calculator handles powers of ten. This tool is for learning and checking, and lab work should follow proper procedures.
Features
- pH, pOH, [H⁺] and [OH⁻] in both directions
- Strong acids and bases
- Weak acids and bases from Ka or Kb
- Percent ionisation for weak species
- Acid, neutral or basic label
- Water correction for very dilute solutions
- Runs in your browser
Frequently asked questions
How do you calculate pH?
pH is the negative base-10 logarithm of the hydrogen ion concentration in moles per litre: pH = -log10[H⁺]. A solution with [H⁺] of 0.001 M has a pH of 3. Each step of one pH unit is a tenfold change in concentration.
What is the relationship between pH and pOH?
In water at 25 degrees Celsius, pH plus pOH equals 14. If the pH is 3, the pOH is 11. The pOH comes from the hydroxide ion concentration in the same way: pOH = -log10[OH⁻].
How do I find the pH of a strong acid?
A strong acid such as hydrochloric acid ionises completely, so [H⁺] equals the acid concentration. A 0.01 M solution of HCl has a pH of 2. For an acid that gives two hydrogen ions per formula, like sulfuric acid in its first approximation, double the concentration.
How do I find the pH of a weak acid?
Solve Ka = x² / (c − x) for the hydrogen ion concentration x, where c is the starting concentration. The calculator solves this quadratic exactly. For 0.1 M acetic acid with Ka of 1.8 × 10⁻⁵, the pH is about 2.88 and about 1.3 percent of the molecules ionise.
Why isn't the pH of 1e-8 M HCl equal to 8?
An acid cannot make a solution basic. At very low concentrations, water's own hydrogen ions (1 × 10⁻⁷ M) are significant, so the total is a little above that. The pH of 1e-8 M HCl is about 6.98. The calculator allows for this.
Does it work at other temperatures?
No. It uses the value of the water ion product at 25 degrees Celsius, where the pH of pure water is 7. At other temperatures neutral pH shifts slightly, and concentrated solutions also depart from ideal behaviour.