pH Calculator
pH, pOH and both ion concentrations, on a log scale.
pH, pOH and both ion concentrations, on a log scale. pH = −log₁₀[H⁺], so the scale is logarithmic: each whole unit is a tenfold change in concentration.
pH
3
acidic · pOH 11
pH = −log₁₀[H⁺], so the scale is logarithmic: each whole unit is a tenfold change in concentration. pH 4 is ten times more acidic than pH 5 and a hundred times more than pH 6, which is why small-sounding pH shifts matter so much in soil, water and blood. The 14 in pH + pOH is temperature dependent — at 50 °C neutral water sits nearer 6.6 while still being perfectly neutral.
How the pH Calculator works
pH = −log₁₀[H⁺], so the scale is logarithmic: each whole unit is a tenfold change in concentration. pH 4 is ten times more acidic than pH 5 and a hundred times more than pH 6, which is why small-sounding pH shifts matter so much in soil, water and blood.
Also known as: poh calculator · hydrogen ion concentration calculator · how to calculate ph · ph formula
A logarithmic scale
pH = −log₁₀[H⁺], so each whole unit is a tenfold change in concentration. pH 4 is ten times more acidic than pH 5 and a hundred times more than pH 6.
That compression is why pH shifts which sound small are not. A lake dropping from pH 6 to pH 5 has become ten times more acidic, which is the difference between healthy and hostile for most fish.
Why 7 is neutral, and when it is not
Pure water dissociates slightly, producing equal amounts of H⁺ and OH⁻. At 25 °C that equilibrium gives 10⁻⁷ M of each, which is where the 7 comes from.
Raise the temperature and water dissociates more, so both concentrations rise and neutral pH falls. At 50 °C neutral water sits nearer 6.6 — still perfectly neutral, because the two ions remain equal, but no longer at 7.
The pH + pOH = 14 relationship carries the same caveat. It is a 25 °C convention rather than a law, and the sum shifts with temperature for the same reason.
Where to go next
The pH question rarely arrives on its own. These are the ones that usually come with it:
- Molarity Calculator — Moles per litre of solution — not of solvent.
- Dilution Calculator — M₁V₁ = M₂V₂, with the diluent volume you actually need.
- Percent Ionization Calculator — Shows the 5% rule failing instead of describing it.
- Speed Distance Time Calculator — Any one of the three from the other two, in four units.
Frequently asked questions
How do I calculate pH?
Take the negative base-10 logarithm of the hydrogen ion concentration. A concentration of 10⁻³ M gives a pH of 3.
What is pOH?
The same measure for hydroxide ions. At 25 °C, pH + pOH = 14, so a pH of 3 means a pOH of 11.
Why is pH 7 neutral?
Because pure water at 25 °C dissociates just enough to give 10⁻⁷ M of each ion. That 7 is temperature dependent — at 50 °C neutral water sits nearer 6.6 while remaining perfectly neutral.
Can pH go below 0 or above 14?
Yes. Concentrated strong acids can reach negative pH and concentrated bases can exceed 14. The 0–14 range is where most aqueous chemistry happens, not a hard limit.
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Related calculators
Molarity Calculator
Moles per litre of solution — not of solvent.
OpenDilution Calculator
M₁V₁ = M₂V₂, with the diluent volume you actually need.
OpenPercent Ionization Calculator
Shows the 5% rule failing instead of describing it.
OpenSpeed Distance Time Calculator
Any one of the three from the other two, in four units.
Open