pH Calculator
Choose what you know: the pH itself, the hydrogen ion concentration, or the strength of an acid or base. You get the pH, pOH and both ion concentrations.
Values at 25 °C, where pH + pOH = 14. You can type small numbers like 0.00001 or 1e-5.
Worked examples
Strong acid
0.1 mol/L hydrochloric acid
pH 1
-log10(0.1).
Weak acid
0.1 mol/L acetic acid, Ka 1.8e-5
pH 2.88
Hydrogen ions 1.33e-3 mol/L.
About pH Calculator
The pH scale measures how acidic or basic a solution is. It is the negative base ten logarithm of the hydrogen ion concentration in moles per litre. Pure water at 25 degrees Celsius has a pH of 7, acids are below 7 and bases above. Each step of one pH unit is a tenfold change in hydrogen ion concentration, so a pH of 3 is a hundred times more acidic than a pH of 5. This calculator converts between pH, pOH and both ion concentrations.
It can also start from the strength of a strong acid or base, where the acid is taken as fully ionised. For a weak acid, give the concentration and its acid dissociation constant Ka, and the page solves the quadratic equation for the hydrogen ion concentration without approximation. Weak bases work the same way with Kb. All values are for 25 degrees Celsius, where the ion product of water is 1.0 times ten to the minus fourteen.
How to use pH Calculator
- Choose what you know: pH, pOH, an ion concentration, or an acid or base strength.
- Enter the value, using scientific notation such as 1e-5 for small concentrations.
- For a weak acid or base, also enter Ka or Kb.
- Read the pH, pOH and both ion concentrations.
Common uses
- Chemistry homework on acids and bases
- Finding the pH of a strong acid like hydrochloric acid
- Estimating the pH of vinegar or ammonia solutions
- Converting a hydrogen ion concentration to pH
Frequently asked questions
What is the pH formula?
pH = -log10 of the hydrogen ion concentration in moles per litre. A concentration of 0.001 mol/L gives a pH of 3.
How is the pH of a weak acid found?
From the equilibrium x squared / (C - x) = Ka. The page solves the quadratic for x exactly, so 0.1 molar acetic acid with Ka of 1.8e-5 gives a pH of 2.88.
What is the relation between pH and pOH?
At 25 degrees Celsius pH + pOH = 14, because the product of the hydrogen and hydroxide concentrations is 1.0e-14.
Why is the strong acid result less accurate at very low concentrations?
Near 1e-7 mol/L the hydrogen ions from water itself become significant, so the simple assumption that the acid supplies all of them breaks down.