About pH Calculator
pH is a logarithmic scale — each whole step is a tenfold change in hydrogen ion concentration, which is precisely why it exists: raw hydrogen ion concentrations in ordinary solutions span many orders of magnitude, from around 1 mol/L in a strong acid down to 10⁻¹⁴ mol/L in a strong base, and nobody wants to work with numbers that small directly. pH = -log₁₀[H⁺] compresses that whole range into the familiar 0–14.
pOH is the same idea for hydroxide ions, and the two are linked: pH + pOH = 14 (at 25°C), because the product of hydrogen and hydroxide ion concentrations in water is a near-constant, Kw = 1.0 × 10⁻¹⁴. That relationship is what makes any one of the four values — pH, pOH, [H⁺], [OH⁻] — enough to derive all the others, which is exactly what this does.
The pH + pOH = 14 relationship is specifically a 25°C figure, not a universal constant — Kw shifts with temperature, so it holds well at typical room/lab temperature and drifts slightly outside that range. For anything requiring real precision at an unusual temperature, that's worth accounting for separately.
How to use the pH calculator
Choose what you know
pH, pOH, [H⁺] or [OH⁻].
Enter the value
A number 0–14 for pH/pOH, or a molar concentration for [H⁺]/[OH⁻].
Read the rest
All four values, plus an acidic-to-basic classification.