Math
pH Calculator
Compute pH from hydrogen ion concentration, from pOH, or from a weak acid/base concentration with Ka or Kb. Returns pH, pOH, [H+], [OH-], and an acidic/neutral/basic verdict.
Compute pH
Use scientific notation like 1e-7 for 10^-7 mol/L.
pH
4
acidic
pOH
10
[H+] (mol/L)
1.000e-4
[OH-] (mol/L)
1.000e-10
Frequently Asked Questions about the pH Calculator
Why is the pH scale logarithmic?
pH is defined as -log10([H+]), so each whole-number step represents a 10x change in hydrogen ion concentration. A pH 4 solution has 1,000 times more H+ ions than a pH 7 solution. That compression is why a shift of even half a unit can matter enormously in chemistry, biology, and industry.
What is the difference between a weak and a strong acid?
A strong acid like HCl dissociates completely in water, so [H+] equals 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 uses the sqrt(Ka x C) approximation for weak acids and the analogous sqrt(Kb x C) for weak bases.
Why does pH plus pOH always equal 14?
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.
What are typical pH values of everyday substances?
Stomach acid is near pH 1.5, lemon juice near 2, black coffee near 5, pure water 7, blood 7.4, baking soda solution 9, household ammonia 11, and oven cleaner near 13. Anything below 7 is acidic, above 7 is basic, and exactly 7 is neutral.
When does the sqrt(Ka x C) approximation break down?
The approximation assumes the fraction of acid that dissociates is small, typically below about 5%. For very dilute solutions or moderately strong weak acids where Ka is not much smaller than C, the error grows. In those cases, solve the full quadratic: x^2 + Ka x x - Ka x C = 0. The quadratic gives the more accurate [H+].