pH Calculator

Convert between pH and hydrogen-ion concentration — go from a pH value to [H⁺]/[OH⁻], or from a known concentration to pH and pOH.

How this works: pH = −log₁₀[H⁺], and at 25°C, pH + pOH = 14 (since K_w = 10⁻¹⁴ = [H⁺][OH⁻]). This calculator treats the concentration you enter as the actual ion concentration in solution — accurate for strong acids/bases, which fully dissociate. Weak acids/bases only partially dissociate, so their pH depends on the acid/base dissociation constant (K_a/K_b), not just the starting concentration.

pH value

1.00 × 10⁻⁷ mol/L

[H⁺] concentration

1.00 × 10⁻⁷ mol/L

[OH⁻] concentration

7.00

pOH

Neutral

Nature of solution

Concentration of ion in solution

0.00

14.00

pOH

1.00 × 10⁰ mol/L

[H⁺] concentration

1.00 × 10⁻¹⁴ mol/L

[OH⁻] concentration

Assumes full dissociation at 25°C (K_w = 1.0×10⁻¹⁴). Weak acids/bases, mixtures, and other temperatures will shift the real pH — use a calibrated pH meter or test strips for an actual sample. Not lab or medical guidance. See Terms.

FAQ

What is pH, exactly?

pH is −log₁₀ of the hydrogen-ion concentration in mol/L. The scale runs roughly 0–14 at 25°C: below 7 is acidic, above 7 is basic (alkaline), and 7 is neutral (pure water).

Does this work for weak acids like vinegar or citric acid?

Only if you already know the actual [H⁺] concentration in the solution. Weak acids don't fully dissociate, so their real [H⁺] is lower than their labelled molarity would suggest for a strong acid — working that out needs the acid's K_a and an equilibrium calculation, which this tool doesn't do.

Why does pH + pOH always equal 14 here?

Because water's ion product K_w = [H⁺][OH⁻] = 1.0×10⁻¹⁴ at 25°C, and taking −log₁₀ of both sides gives pH + pOH = 14. At other temperatures K_w shifts slightly, so that total isn't exactly 14.

FAQ

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