pH Calculator
Use pH = −log₁₀([H⁺]/1 mol/L) and pH + pOH = 14. Concentrations approximate activities in this educational model.
Ideal dilute aqueous model at 25 °C, pKw = 14. Actual pH is defined by hydrogen ion activity; temperature and non-ideal behavior change the relationship. pH and pOH are logarithms and can be negative or exceed 14.
pH = −log10(aH+) ≈ −log10([H+]/1 mol/L)
pH + pOH = pKw ≈ 14 (25 °C)
Ka and initial acid/base molarity are not supported. Enter an equilibrium hydrogen ion concentration, not the total amount of dissolved acid. This tool does not estimate activity coefficients or calculate a temperature-dependent pKw.
Converted values
- pH
- pOH
- [H+] (mol/L)
- [OH−] (mol/L)
Definition and reference: IUPAC Gold Book — pH; OpenStax Chemistry 2e §14.2
A QUICK WALKTHROUGH
How to use this tool
- Choose [H⁺], pH, or pOH as the known quantity.
- Enter a value; [H⁺] must be positive and finite.
- Calculate pH, pOH, [H⁺] and [OH⁻].
Definition and assumptions
Ideal dilute aqueous model at 25 °C, pKw = 14. Actual pH is defined by hydrogen ion activity; temperature and non-ideal behavior change the relationship. pH and pOH are logarithms and can be negative or exceed 14.
GOOD TO KNOW
Common questions
Does this predict pH from the amount of an acid or base?
No. This tool only converts a supplied pH, pOH, or hydrogen ion concentration. It does not model dissociation, buffering, mixtures or laboratory procedures. Ka and initial acid/base molarity are not supported. Enter an equilibrium hydrogen ion concentration, not the total amount of dissolved acid. This tool does not estimate activity coefficients or calculate a temperature-dependent pKw.