pH Calculator
Enter a hydrogen-ion concentration in mol/L to get the pH on the 0–14 scale — plus the matching pOH — and see whether a solution is acidic, neutral, or basic.
pH and pOH at once
Enter the hydrogen-ion concentration [H⁺] in mol/L and the calculator returns the pH (−log₁₀[H⁺]) and the pOH (14 − pH) together.
Use mol/L
The concentration must be in moles per litre. Pure neutral water is 0.0000001 (1 × 10⁻⁷) mol/L, which gives a pH of 7.
What is pH?
The measure of acidity
pH is a measure of how acidic or basic a water-based solution is, on a scale that usually runs from 0 to 14. It is defined as the negative base-10 logarithm of the hydrogen-ion concentration [H⁺]. A pH of 7 is neutral, values below 7 are acidic, and values above 7 are basic (alkaline). The pH calculator turns one measurement — the hydrogen-ion concentration in moles per litre — into the pH, alongside the pOH (14 minus the pH). It is the number behind water quality, blood chemistry, soil testing, and countless laboratory reactions.
Enter a hydrogen-ion concentration in mol/L to get the pH on the 0–14 scale and the pOH instantly.
The pH is the negative base-10 logarithm of the hydrogen-ion concentration, and the pOH is simply 14 minus the pH at 25 °C.
pH = −log₁₀([H⁺])Because the scale is logarithmic, each whole pH unit represents a tenfold change in [H⁺]: a solution at pH 3 has ten times more hydrogen ions than one at pH 4 and a hundred times more than pH 5. Enter the concentration in mol/L and the pH comes back on the 0–14 scale, with the pOH as 14 − pH.
Suppose a solution has a hydrogen-ion concentration of 0.001 mol/L (1 × 10⁻³).
Take the base-10 logarithm
log₁₀(0.001) = −3 — the power of ten that gives 0.001.
Negate it
pH = −(−3) = 3 — the solution is acidic.
Find the pOH
pOH = 14 − 3 = 11 — the matching hydroxide measure at 25 °C.
The pH tells you at a glance where a solution sits between acidic and basic. A pH of exactly 7 (the [H⁺] of pure water, 1 × 10⁻⁷ mol/L) is neutral; anything below 7 is acidic and anything above 7 is basic, with stomach acid near 1–2, lemon juice near 2, pure water at 7, baking soda near 9, and household bleach near 13. The crucial point is that the scale is logarithmic: dropping from pH 5 to pH 3 is not "a little more acidic" — it is a hundredfold increase in hydrogen ions, because each unit is a factor of ten. The pOH mirrors this from the hydroxide side and always adds up with the pH to 14 in water at 25 °C, so a pH of 3 implies a pOH of 11. That relationship is why knowing one of the two immediately gives you the other.
The formula is exact, but a couple of practical points are worth keeping in mind.
Dilute aqueous solutions at 25 °C
This calculator uses pH = −log₁₀([H⁺]) with the concentration in mol/L and the pOH = 14 − pH relationship, which assumes a dilute aqueous solution at 25 °C. In concentrated solutions the true value depends on ion activity rather than concentration, and the neutral point and the 14 sum shift with temperature. The concentration must be greater than zero — the logarithm of zero or a negative number is undefined.