pH Calculator

This calculator uses the common concentration approximation pH = -log10([H+]), so each whole-number step represents a 10x change in hydrogen ion concentration. The thermodynamic definition uses hydrogen-ion activity, which can differ from concentration in non-ideal solutions.

A strong acid like HCl dissociates completely in water, so [H+] is approximately the molar concentration you started with. A weak acid like acetic acid dissociates only partially, so you need its acid dissociation constant Ka to find [H+]. This calculator solves the weak-electrolyte equilibrium and includes water autoionization.

At 25 degrees C, water self-ionizes so that [H+] x [OH-] = 1.0 x 10^-14 (Kw). Taking -log10 of both sides gives pH + pOH = 14. This calculator applies that relationship directly, so the identity holds for all three input modes. At temperatures other than 25 C, Kw shifts and the sum deviates slightly from 14.

Approximate values include lemon juice near pH 2, black coffee near 5, pure water near 7 at 25 degrees C, blood near 7.4, baking soda solution near 9, and household ammonia near 11. Neutral pH changes with temperature, so the simple below-7 acidic and above-7 basic rule assumes about 25 degrees C.

The approximation assumes that little acid dissociates and that the acid contributes far more [H+] than water does. It becomes inaccurate when Ka is not much smaller than the starting concentration or when the solution is very dilute. This calculator instead solves [H+] = KaC / (Ka + [H+]) + Kw / [H+], which includes both acid dissociation and water autoionization.