pH calculator
Enter any one of pH, pOH, [H⁺] or [OH⁻] to get the other three, or find the pH of a weak acid or base from its Ka or Kb. Answers follow the log rule for significant figures.
The four equations
- pH = −log[H⁺], and [H⁺] = 10^−pH
- pOH = −log[OH⁻], and [OH⁻] = 10^−pOH
- pH + pOH = 14.00 at 25 °C
- [H⁺][OH⁻] = Kw = 1.0 × 10⁻¹⁴ at 25 °C
Significant figures in pH
The digits before the decimal point in a pH only locate the power of ten, so the number of decimal places in pH equals the significant figures in the concentration.
Example: [H⁺] = 2.5 × 10⁻⁴ M has 2 significant figures, so pH = 3.60, with two decimal places.
Weak acids and bases
Weak acids only partly ionize, so [H⁺] isn’t equal to the acid concentration. Set up Ka = x² / (C − x) and solve for x = [H⁺]. This calculator solves the quadratic exactly and reports the percent ionized.
Example: 0.10 M acetic acid, Ka = 1.8 × 10⁻⁵: x = 1.33 × 10⁻³ M, so pH = 2.88 (1.33% ionized).
The “x is small” shortcut
Many classes drop the x in C − x when the acid is less than 5% ionized. The calculator tells you whether that shortcut is valid for your numbers, so you know when your teacher expects the full quadratic. For buffers and titrations, ask the chemistry solver.
Common questions.
Can pH be negative or above 14?
Yes, for very concentrated strong acids or bases (above 1 M). Most homework problems stay between 0 and 14.
How do I find the pH of a strong acid?
Strong acids ionize completely, so [H⁺] equals the acid concentration for monoprotic acids like HCl. Enter it as [H⁺]. For a strong base like NaOH, enter its concentration as [OH⁻].
Does temperature matter?
Yes. Kw changes with temperature, so pH + pOH equals 14.00 only at 25 °C. This calculator assumes 25 °C, as most courses do.
What’s the difference between [H⁺] and [H₃O⁺]?
They mean the same thing in these calculations. H⁺ is shorthand for the hydronium ion, H₃O⁺, that forms in water.