🧪 pH Calculator
Enter any one value — pH, pOH, [H⁺] or [OH⁻] — at 25°C, and the others are calculated from it.
Four ways of saying the same thing
pH is a logarithm of the hydrogen ion concentration, and at 25°C water's ion product ties the acid and base halves together:
pH = −log₁₀[H⁺] [H⁺] = 10⁻ⁿₕ
pOH = −log₁₀[OH⁻] pH + pOH = 14
[H⁺] × [OH⁻] = 1.0 × 10⁻¹⁴Pick what you know from the dropdown and the other three follow. Lemon juice at pH 2.4 gives [H⁺] = 3.981 × 10⁻³ mol/L, a pOH of 11.6 and [OH⁻] of 2.512 × 10⁻¹². The default pH 7 is the balance point, where both concentrations are 1.0 × 10⁻⁷.
Entering values and the two assumptions
- Concentrations take scientific notation. Type 1e-7 or 0.0000001; either parses. They must be above zero, since the log of zero is undefined.
- pH itself may be negative or above 14. Concentrated acids genuinely reach those readings, so the tool does not clamp the scale.
- Each whole pH unit is a factor of ten, which is why pH is reported to three decimals and concentrations in scientific notation.
Frequently asked questions
What is the pH of 0.001 M hydrochloric acid?
3. HCl dissociates completely, so the hydrogen ion concentration is the same 0.001 mol/L, and the negative log of 0.001 is 3.
How much more acidic is pH 3 than pH 5?
100 times. The scale is logarithmic, so each whole step multiplies the hydrogen ion concentration by ten and two steps make a hundred.
Do pH and pOH always add up to 14?
Only at 25 degrees Celsius, which is what this page assumes. The sum tracks the ion product of water, and that rises with temperature.