pH Calculator
Calculate pH, pOH and hydroxide concentration from the hydrogen-ion concentration of a solution.
How it works
pH = −log₁₀[H⁺] · pOH = 14 − pH · [OH⁻] = 10^(−pOH)
pH measures how acidic or basic a solution is on a 0–14 scale. It is the negative base-10 logarithm of the hydrogen-ion concentration, so each whole pH unit is a tenfold change in acidity. Below 7 is acidic, 7 is neutral, above 7 is basic.
Worked example
A [H⁺] of 1×10⁻⁴ mol/L gives pH = −log₁₀(0.0001) = 4, which is acidic.
Frequently asked questions
Why is the pH scale logarithmic?
Hydrogen-ion concentrations span many orders of magnitude, so a log scale compresses them into a readable 0–14 range. Each unit is a 10× change in concentration.
What is pOH?
pOH measures hydroxide-ion concentration the same way pH measures hydrogen ions. At 25°C, pH + pOH always equals 14.
Can pH go below 0 or above 14?
Yes, for very concentrated strong acids or bases the value can fall outside 0–14, though such solutions are unusual.
Related calculators
Educational estimate. Check the formula and units for your exact case.