pH from Concentration Calculator
pH measures how acidic or basic a solution is on a logarithmic scale set by the hydrogen ion concentration. This calculator takes the hydrogen ion concentration in moles per litre and returns the pH, the pOH, and the hydroxide ion concentration. The pH to pOH conversion uses the water dissociation constant at 25 degrees Celsius. Enter the H+ molarity to see where the solution sits on the 0 to 14 scale.
pH formula
pH = -log10[H+]
pOH = 14 - pH (at 25 deg C)
[OH-] = 10 ^ (-pOH)
pH below 7 is acidic, exactly 7 is neutral, and above 7 is basic at 25 degrees Celsius. The 14 comes from Kw = 1e-14, the water dissociation constant at that temperature.
Worked example
A hydrogen ion concentration of 0.001 molar (1e-3): pH = -log10(0.001) = 3.00, so the solution is acidic. pOH = 14 - 3 = 11.00. [OH-] = 10^(-11) = 1.00e-11 molar.
pH from concentration: frequently asked questions
How do you calculate pH from concentration?
pH is the negative base-10 logarithm of the hydrogen ion concentration in moles per litre: pH = -log10[H+]. A hydrogen ion concentration of 1e-3 molar gives pH = -log10(0.001) = 3.
How are pOH and hydroxide concentration found?
At 25 degrees Celsius pH plus pOH equals 14, so pOH = 14 minus pH. The hydroxide concentration is 10 raised to the power minus pOH. These follow from the water dissociation constant Kw = 1e-14 at 25 degrees Celsius.
Does this assume a temperature?
The pH to pOH relationship uses Kw = 1e-14, which holds at 25 degrees Celsius. Kw changes with temperature, so at other temperatures the sum of pH and pOH differs from 14. The pH from H+ value itself does not depend on temperature.
Sources
- NIST: SI units, including the mole and litre.
- pH = -log10[H+] and Kw = 1e-14 at 25 deg C are standard definitions in aqueous chemistry.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.