What does pH measure?

The concentration of hydrogen ions, on a negative logarithmic scale. pH 7 is neutral at 25°C, lower is acidic and higher is alkaline.

Why is a pH change of 1 significant?

Because the scale is logarithmic. One unit is a tenfold change in hydrogen ion concentration, so pH 3 is a hundred times more acidic than pH 5 rather than slightly more.

How do I convert pH to hydrogen ion concentration?

Concentration equals 10 to the power of negative pH, in moles per litre. pH 3 is 10⁻³, or 0.001 mol/L. The reverse is the negative logarithm of the concentration.

What is the relationship between pH and pOH?

They sum to 14 in water at 25°C, because the ion product of water is fixed at that temperature. At other temperatures the sum differs, which is why neutral is not always exactly 7.

Is neutral pH always 7?

Only at 25°C. Pure water at 50°C has a neutral pH of about 6.6 — still neutral, because hydrogen and hydroxide concentrations are still equal, even though the number is below 7.

pH Calculator

Convert between pH, pOH and ion concentration, with a visual scale.

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About the pH Calculator

pH is the negative base-10 logarithm of hydrogen ion concentration. Being logarithmic is the fact that matters: each whole pH unit is a tenfold change in acidity, so pH 3 is ten times more acidic than pH 4 and a hundred times more than pH 5.

That is why small-sounding pH shifts are large. Ocean pH falling from 8.2 to 8.1 sounds trivial and represents about a 26 percent increase in hydrogen ion concentration. It is also why diluting a strong acid tenfold moves the pH by exactly one unit.

pH and pOH always sum to 14 in water at 25°C, because the ion product of water is fixed. That relationship holds only at that temperature — pure water at 50°C has a neutral pH of about 6.6, which is still neutral even though it is below 7.

How it works

Enter any one of pH, pOH, [H⁺] or [OH⁻].
The other three follow, since pH + pOH = 14 at 25 °C.
The scale shows where the value sits and what everyday substance is nearby.

Frequently asked questions

What does pH measure?: The concentration of hydrogen ions, on a negative logarithmic scale. pH 7 is neutral at 25°C, lower is acidic and higher is alkaline.
Why is a pH change of 1 significant?: Because the scale is logarithmic. One unit is a tenfold change in hydrogen ion concentration, so pH 3 is a hundred times more acidic than pH 5 rather than slightly more.
How do I convert pH to hydrogen ion concentration?: Concentration equals 10 to the power of negative pH, in moles per litre. pH 3 is 10⁻³, or 0.001 mol/L. The reverse is the negative logarithm of the concentration.
What is the relationship between pH and pOH?: They sum to 14 in water at 25°C, because the ion product of water is fixed at that temperature. At other temperatures the sum differs, which is why neutral is not always exactly 7.
Is neutral pH always 7?: Only at 25°C. Pure water at 50°C has a neutral pH of about 6.6 — still neutral, because hydrogen and hydroxide concentrations are still equal, even though the number is below 7.

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