pH Calculator

How to use this calculator

Only one input is needed, because at 25 °C the four quantities are locked together by the ion product of water. Enter a pH-meter reading to get the ion concentrations, or enter a concentration (for a strong acid or base, the concentration of the acid or base itself) to get pH. Very small concentrations can be typed in scientific notation, such as 3.2e-9.

Concentration fields accept M, mM, µM and nM. For a strong monoprotic acid like HCl, [H⁺] equals the acid concentration; for a strong base like NaOH, [OH⁻] equals the base concentration. Weak acids only partly dissociate, so [H⁺] must first be found from the acid dissociation constant K_a.

The formula

pH = −log₁₀[H⁺]

pOH = −log₁₀[OH⁻]

pH + pOH = 14 (25 °C)

pH: ph
pOH: poh
[H⁺]: hydrogen ion concentration
[OH⁻]: hydroxide ion concentration

Worked example

A dilute hydrochloric acid solution has a hydrogen ion concentration of 1 mM (0.001 mol/L).

pH = −log₁₀(0.001) = 3.
pOH = 14 − 3 = 11.
[OH⁻] = 10⁻¹¹ mol/L.

pH 3 is strongly acidic, roughly the acidity of vinegar or soda.

Assumptions and limits

Temperature is 25 °C, where K_w = 1.0 × 10⁻¹⁴. At 37 °C neutral water has pH ≈ 6.8.
Concentrations stand in for activities, which is accurate for dilute solutions (below roughly 0.1 M).
Very dilute acids (below 10⁻⁶ M) are dominated by water self-ionization; the simple formula will understate pH.

Frequently asked questions

Can pH be negative?

Yes. Concentrated strong acids above 1 M have negative pH; 10 M HCl is about pH −1 on the concentration scale.

Is pH 7 always neutral?

Only at 25 °C. Neutral means [H⁺] = [OH⁻], which occurs at pH 7.47 at 0 °C and about pH 6.14 at 100 °C.

How much more acidic is pH 3 than pH 5?

The scale is logarithmic, so each unit is a factor of ten. pH 3 has 100 times the hydrogen ion concentration of pH 5.