Donaldson R F pH & pOH Calculator
Enter any one of pH, pOH, [H⁺] or [OH⁻] to get the other three.
How this is worked out
pH is defined as −log₁₀[H⁺] and pOH as −log₁₀[OH⁻], where [H⁺] and [OH⁻] are the hydrogen and hydroxide ion concentrations in moles per litre. For dilute aqueous solutions at 25°C, water's ion-product constant Kw = [H⁺][OH⁻] = 1.0×10&supminus;¹⁴ links all four quantities together, giving the familiar pH + pOH = 14. A pH of 7 is neutral; lower is acidic, higher is alkaline (basic).
This tool assumes 25°C and a dilute solution, the standard simplification for an introductory calculator — Kw actually changes with temperature (it's larger at higher temperatures), and very concentrated solutions need activity corrections this simple formula doesn't include.
Frequently asked questions
Can pH be negative or above 14?
Yes — very concentrated strong acids can have a pH below 0, and very concentrated strong bases can go above 14; the 0–14 range just covers the dilute solutions encountered in most everyday and school contexts.
Why a logarithmic scale?
Because [H⁺] in real solutions spans many orders of magnitude (from about 1 mol/L in a strong acid down to 10&supminus;¹⁴ mol/L in a strong base) — a log scale compresses that huge range into small, easy-to-compare numbers, at the cost of each whole pH unit meaning a tenfold change in acidity.
Related tools: Molar Mass Calculator, Chemical Equation Balancer, Periodic Table Lookup.
Source / Last verified: standard general/analytical chemistry definitions (pH = −log[H⁺], water's ion-product constant Kw = 1.0×10&supminus;¹⁴ at 25°C) — read/checked 1 October 2026. This tool reproduces the standard textbook identity pH + pOH = 14 at 25°C for any input, and [H⁺] = 10&supminus;ⁿ = 1.00×10&supminus;ⁿ mol/L exactly at pH 7 (neutral water).