pH Calculator

Written byOleh Soloninupdated 5 October 2026

What you know

H⁺ concentration, mol/l

Result

pH3
Character of the solutionacidic
pOH11
H⁺ concentration, mol/l1 × 10⁻³
OH⁻ concentration, mol/l1 × 10⁻¹¹
Degree of dissociation, %weak acid mode only

A logarithmic scale defeats intuition

Works out pH and pOH from a hydrogen ion concentration or from the dissociation constant of a weak acid.

pH is the negative base-ten logarithm of the hydrogen ion concentration. The logarithm makes the scale deceptive: the gap between pH 3 and pH 4 is not 'a little', it is tenfold in acidity, and between pH 3 and pH 6 it is a thousandfold.

Strong acids are straightforward: they dissociate completely, so the ion concentration equals the acid concentration. Weak acids dissociate partially, and for those the approximation runs through the constant — the ion concentration is the square root of the constant times the concentration.

How to use

Choose the starting point: ion concentration, a known pH, or a weak acid with its constant.
Enter the value you have.
Read pH and pOH along with both ion concentrations.

A logarithmic scale defeats intuition

pH is the negative base-ten logarithm of the hydrogen ion concentration. A concentration of 0.001 mol/l gives pH 3, and pOH completes it to 14. One step on the scale is a tenfold change in concentration.

So stomach acid at about pH 2 is not slightly more acidic than water but a hundred thousand times so. For the same reason, mixing equal volumes of an acid and a base rarely neutralises anything — the amounts of substance have to match, not the volumes.

Worked out with this calculator at 25 °C.
H⁺ concentration, mol/l	pH	pOH	Character
0.1	1	13	strongly acidic
0.001	3	11	acidic
0.0000001	7	7	neutral
0.00000000001	11	3	alkaline

What this calculator shows

pH and pOH, which always sum to 14
Hydrogen and hydroxide ion concentrations
The degree of dissociation in weak-acid mode

What to keep in mind

pH plus pOH equals 14 at 25 °C; at other temperatures the ion product of water shifts.
The weak-acid approximation holds while dissociation stays under about five percent.

FAQs

How do I calculate pH?

Take the negative base-ten logarithm of the hydrogen ion concentration. For 0.001 mol/l that gives pH 3.

How are pH and pOH related?

They sum to 14 at 25 degrees. pH 3 means pOH 11 and a hydroxide concentration of 10⁻¹¹ mol/l.

What separates a strong acid from a weak one?

A strong acid dissociates completely, so ion concentration equals acid concentration. A weak acid dissociates only partly — how far depends on Ka and concentration, 1.32% for 0.1 M acetic acid — and its pH runs through Ka.

Can pH go below zero?

Yes, in concentrated acids. The 0 to 14 range is a convenient span for aqueous solutions, not a physical limit.

What is the pH at a hydrogen ion concentration of 0.001 mol/l?

Three, from pH = −log 0.001. The pOH is 11, because pH and pOH sum to 14 in water at 25 °C.

How much more acidic is pH 3 than pH 6?

A thousand times. Each unit on the scale is a factor of ten in hydrogen ion concentration.

Why is neutral pH not always exactly seven?

Seven is neutral at 25 °C. Heating raises the ionic product of water and moves the neutral point down — near 6.5 at 60 °C, with the water no less neutral for it.

Worked example

Acetic acid

Input: Ka = 1.75 × 10⁻⁵, concentration 0.1 mol/l

Output: pH 2.88, dissociation 1.32%

Note: Hydrochloric acid at the same concentration would give pH exactly 1 — nearly eighty times more hydrogen ions. That gap is the whole difference between a strong and a weak acid.

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Result

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