pH Calculator

Enter a pH, or the hydrogen ion concentration, to find the other values and whether the solution is acidic or basic.

This result is an estimate only. Double-check important figures before relying on them.

How it is calculated

pH = −log₁₀[H⁺], and [H⁺] = 10^(−pH)
pOH = 14 − pH
[OH⁻] = 10^(−pOH)
At 25 °C, [H⁺] × [OH⁻] = 1 × 10⁻¹⁴

For logarithms in general, use the logarithm calculator. To write small numbers, use the scientific notation calculator.

Worked example

A solution with pH 3:

[H⁺] = 1 × 10⁻³ mol/L
pOH = 14 − 3 = 11, so [OH⁻] = 1 × 10⁻¹¹ mol/L
It is acidic

A [H⁺] of 0.001 mol/L gives −log₁₀(0.001) = 3.

Why your number may differ

This uses the 25 °C relationship and treats concentration as activity, which is a fair approximation for dilute solutions. Very strong or very concentrated solutions can behave differently.

Frequently asked questions

What is pH?

pH measures how acidic or basic a solution is: pH = −log₁₀[H⁺]. Below 7 is acidic, 7 is neutral and above 7 is basic.

What is pOH?

pOH = −log₁₀[OH⁻]. At 25 °C, pH + pOH = 14.

Why 25 °C?

The relationship pH + pOH = 14 holds at 25 °C. The value changes slightly with temperature, so the neutral point is not exactly 7 at other temperatures.