pH Calculator
Convert between [H⁺], pH, pOH and [OH⁻] and classify the acidity.
Results update as you type.
About this calculator
The pH scale measures how acidic or basic a solution is, defined as the negative base-10 logarithm of the hydrogen-ion concentration: pH = −log₁₀[H⁺], where [H⁺] is in moles per litre. Because it is logarithmic, each whole pH unit is a tenfold change in acidity — a pH 4 solution has ten times the [H⁺] of pH 5 and a hundred times that of pH 6. The scale runs from strongly acidic near 0, through neutral at 7, to strongly basic near 14. This calculator converts a concentration to pH, a pH back to concentration, or starts from pOH.
As a worked example, pure water has [H⁺] = 1 × 10⁻⁷ mol/L, so pH = −log₁₀(10⁻⁷) = 7, which is neutral. A solution with [H⁺] = 10⁻³ mol/L has pH 3 and is acidic; one with pH 11 is strongly basic. The tool also returns pOH and the hydroxide concentration [OH⁻], using the water relationship pH + pOH = 14 at 25 °C — so pH 3 implies pOH 11 and [OH⁻] = 10⁻¹¹ mol/L — and labels the result acidic, neutral or basic.
pH governs a huge range of everyday and industrial processes: the safety of drinking water and swimming pools, soil conditions for gardening, food preservation and brewing, the action of enzymes in the body (blood is tightly held near pH 7.4), and countless chemistry and biology experiments. Note that the 14-point span and the pH + pOH = 14 rule are specific to 25 °C; at other temperatures the neutral point and the sum shift slightly because water’s self-ionisation changes.
Frequently asked questions
What is the pH formula?
pH is the negative base-10 logarithm of the hydrogen-ion concentration, pH = −log₁₀[H⁺]. A concentration of 1×10⁻⁷ mol/L gives a neutral pH of 7.
How are pH and pOH related?
At 25 °C, pH + pOH = 14. So a solution of pH 3 has pOH 11, and its hydroxide concentration is [OH⁻] = 10⁻¹¹ mol/L.
What does the pH scale mean?
Below 7 is acidic, exactly 7 is neutral, and above 7 is basic (alkaline). The scale usually runs 0–14, though very strong acids or bases can fall slightly outside it. The calculator classifies your value as acidic, neutral or basic.
Why is pH logarithmic?
Hydrogen-ion concentrations span many orders of magnitude, so a log scale keeps the numbers manageable. Each unit is a tenfold change: pH 4 is ten times more acidic than pH 5 and a hundred times more than pH 6.
How do I find [H⁺] from a pH value?
Invert the definition: [H⁺] = 10^(−pH). A pH of 3 gives [H⁺] = 10⁻³ = 0.001 mol/L. Choose "from a pH value" mode and the calculator returns the concentration for you.
Does pH change with temperature?
Yes. The neutral point of 7 and the pH + pOH = 14 rule hold at 25 °C; at higher temperatures water ionises more, so neutral pH drops below 7 and the sum falls slightly. This tool uses the standard 25 °C values.
API — use this calculator from code
Call this calculator as a free JSON endpoint — no key required. Send the field values below as query parameters or JSON. Read the full API docs →
Endpoint
GET https://calculator.free/api/v1/ph/curl
curl "https://calculator.free/api/v1/ph/?mode=conc&conc=1e-07&phval=7"JavaScript fetch()
const r = await fetch(
"https://calculator.free/api/v1/ph/?" + new URLSearchParams({
"mode": "conc",
"conc": "0.0000001",
"phval": "7"
}));
const data = await r.json();
console.log(data.results);Results are estimates for general guidance only, not financial, medical or tax advice.