pH Calculator
Convert instantly between pH, pOH, [H⁺] and [OH⁻], or find the pH of a strong or weak acid or base from its concentration.
pH ⇄ pOH ⇄ [H⁺] ⇄ [OH⁻]
pH
pH
acidic
pOH
pOH
H⁺
Hydrogen ion concentration
M
OH⁻
Hydroxide ion concentration
M
Assumes 25 °C (Kw = 1.0 × 10⁻¹⁴).
pH of an acid or base solution
Acetic acid Ka = 1.8e-5, ammonia Kb = 1.8e-5
pH
pH of the solution
Shortcut √(K·C) gives 2.87
%
Percent ionisation
%
Under 5 %: the shortcut is valid
The pH scale is logarithmic: slide and see
[H⁺] = 1 × 10⁻⁴ M. That is 1,000× more H⁺ than pure water. Each step of 1 on the scale is a factor of 10.
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0/1500
pH of everyday substances
Battery acid ≈ 0–1, stomach acid ≈ 1.5–3.5, lemon juice ≈ 2.2, black coffee ≈ 5, pure water 7.0 at 25 °C, blood 7.35–7.45, baking soda solution ≈ 8.3, household ammonia ≈ 11.5, bleach ≈ 12.5. Lemon juice is roughly 100 000 times more acidic than pure water, not three times, because the scale is logarithmic.
Frequently asked questions
How do you calculate pH?
pH = −log₁₀[H⁺], with [H⁺] in mol/L. For [H⁺] = 3.2 × 10⁻⁴ M, pH = −log(3.2 × 10⁻⁴) = 3.49.
How do I get [H⁺] from pH?
[H⁺] = 10^(−pH). pH 3.50 means [H⁺] = 10⁻³·⁵ = 3.16 × 10⁻⁴ M.
Why do pH and pOH add to 14?
Water self-ionises: [H⁺][OH⁻] = Kw = 1.0 × 10⁻¹⁴ at 25 °C. Taking −log of both sides gives pH + pOH = 14. At other temperatures Kw changes, so the sum is not exactly 14.
How do I find the pH of a weak acid?
Set up an ICE table: Ka = x² ÷ (C − x) where x = [H⁺]. This calculator solves the quadratic exactly and also shows the common shortcut x ≈ √(Ka·C), which is fine when less than 5 % of the acid ionises.
How many decimal places should a pH have?
As many as the significant figures in [H⁺]. [H⁺] = 3.2 × 10⁻⁴ (2 s.f.) gives pH 3.49 (2 decimal places). The digits before the point only reflect the power of ten.