pH calculator
Choose what you know and enter the value. The other three follow.
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How do you calculate pH?
pH is the negative base-ten logarithm of the hydrogen ion concentration in moles per litre. A concentration of 0.001 mol/L gives a pH of 3, because the logarithm of 0.001 is −3. At 25 °C, pH and pOH always add to 14, so either one gives you the other.
The scale is logarithmic, which is easy to forget
Each whole pH unit is a tenfold change in hydrogen ion concentration. A solution at pH 4 is ten times more acidic than one at pH 5 and a hundred times more acidic than one at pH 6.
This is why a change from pH 7 to pH 5 is a far bigger event than the two-unit gap suggests, and why pH differences cannot be averaged the way ordinary measurements can. Averaging pH 3 and pH 5 does not give the pH of the mixture.
To combine solutions properly you convert each to a concentration, mix those, and convert back — which is exactly what the concentration fields above are for.
The relationships
Four quantities, any one of which gives the other three at 25 °C.
pH = −log₁₀[H⁺]pOH = −log₁₀[OH⁻][H⁺] = 10^(−pH)[OH⁻] = 10^(−pOH)pH + pOH = 14 (at 25 °C)[H⁺] × [OH⁻] = 1.0 × 10⁻¹⁴
The value of 14 is not a universal constant — it comes from the ion product of water, which is temperature dependent. At 25 °C it is 1.0 × 10⁻¹⁴, giving the familiar 14. At 50 °C it is larger, and neutral water sits at about pH 6.6 rather than 7.
That means neutral does not always mean pH 7. It means [H⁺] equals [OH⁻], which happens at pH 7 only at 25 °C.
Worked example: a 0.005 mol/L strong acid
A strong monoprotic acid at 0.005 mol/L dissociates completely, so the hydrogen ion concentration equals the acid concentration.
- Hydrogen ion concentration0.005 mol/L
- log₁₀(0.005)−2.301
- pH = −(−2.301)2.30
- pOH = 14 − 2.3011.70
- Hydroxide concentration2.00 × 10⁻¹² mol/L
pH 2.30, strongly acidic.
This works because the acid is strong and monoprotic. A weak acid only partially dissociates, so its hydrogen ion concentration is far below its formal concentration and you need the acid dissociation constant to find the pH.
Assumptions and limits
- Assumes 25 °C — the ion product of water, and therefore neutral pH, shifts with temperature
- Assumes ideal behaviour; concentrated solutions need activity rather than concentration
- For weak acids and bases you need the dissociation constant, not just the concentration
- Polyprotic acids release more than one proton and need stepwise treatment
- Very strong solutions can give pH values below 0 or above 14, which are real but rarely useful
Common questions
What is the pH formula?
pH equals the negative base-ten logarithm of the hydrogen ion concentration in moles per litre. To go the other way, hydrogen ion concentration equals ten raised to the power of negative pH.
Why does pH plus pOH equal 14?
Because the ion product of water at 25 °C is 1.0 × 10⁻¹⁴, and taking negative logarithms of both sides of that relationship gives 14. At other temperatures the ion product changes and the sum is no longer exactly 14.
Is pH 7 always neutral?
Only at 25 °C. Neutral means the hydrogen and hydroxide concentrations are equal, and the pH at which that happens shifts with temperature. At 50 °C neutral water sits near pH 6.6, and it is still neutral.
Can pH be negative?
Yes. A hydrogen ion concentration above 1 mol/L gives a negative pH, which happens in concentrated strong acids. The value is real, though at those concentrations activity rather than concentration governs the actual behaviour.
Can I average two pH values?
No. Because the scale is logarithmic, averaging pH values does not give the pH of a mixture. Convert each to a hydrogen ion concentration, combine those, and convert the result back.
Where these numbers come from
- IUPAC — Compendium of Chemical Terminology, pH The formal definition of pH.
- NIST — Ion product of water Temperature dependence of the water ionisation constant.
Last verified 2026-08-01 The method on this page is checked against the sources above at least once a year. Spotted something out of date? Tell us and we will fix it.
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