pH and pOH measure acidity and basicity on a log scale. At 25 °C they satisfy pH + pOH = 14, with [H⁺] = 10⁻ᵖᴴ and [OH⁻] = 10⁻ᵖᴼᴴ. Enter any one of the four and this calculator returns the rest and tells you whether the solution is acidic, neutral or basic.
Reviewed: June 20, 2026 · Author: Naveen P N, Founder — AI Calculator · Verified against: Kw and the log definitions, recomputed in code.
The relationships
The "p" means "−log₁₀ of," so each unit of pH is a tenfold change in [H⁺]. Because water's ion product Kw is 10⁻¹⁴ at 25 °C, the two logs always sum to 14. Acidic solutions have more H⁺ than OH⁻ (pH < 7); basic solutions have more OH⁻ (pH > 7); pure water is neutral with both at 10⁻⁷ M.
Worked examples
pH 3:
[H⁺] = 0.01 M:
pOH 4:
A pH of 8.5 gives pOH 5.5, with [H⁺] ≈ 3.16×10⁻⁹ M and [OH⁻] ≈ 3.16×10⁻⁶ M — basic, since there's a thousand times more hydroxide than hydrogen ion.
Frequently Asked Questions
At 25 °C, pH + pOH = 14, from Kw = [H⁺][OH⁻] = 10⁻¹⁴.
[H⁺] = 10^(−pH). pH 3 → 0.001 M. Reverse: pH = −log₁₀[H⁺].
pOH = −log₁₀[OH⁻]. [OH⁻] = 10⁻⁴ → pOH 4, pH 10.
Below 7 acidic, 7 neutral, above 7 basic (at 25 °C).
Kw changes with temperature, so neutral pH and the sum shift away from 7 and 14.