pH Converter

pH measures acidity or alkalinity on a scale from 0 to 14.

Core formulas:

• pH = -log₁₀[H⁺]
• pOH = -log₁₀[OH⁻]
• pH + pOH = 14 (at 25°C)
• [H⁺] = 10⁻ᵖᴴ mol/L
• [OH⁻] = 10⁻ᵖᴼᴴ mol/L
• [H⁺] × [OH⁻] = 10⁻¹⁴ (water ionization constant at 25°C)

The pH scale, with substances people actually recognise:

Baking soda is worth a note because charts routinely put it at 9 or 10. A saturated sodium bicarbonate solution sits at about 8.3, which is mild enough to be safe on skin. The chemical that actually reaches 10 or 11 in a kitchen is washing soda, sodium carbonate, and that one does burn.

Important: Each whole pH unit represents a 10× change in H⁺ concentration. pH 3 is 10 times more acidic than pH 4. pH 3 is 100 times more acidic than pH 5.

The pH scale comes straight from chemistry: pH is the negative base-10 logarithm of the hydrogen-ion concentration. That logarithm is why the scale behaves as the page describes, with each whole step a tenfold change in acidity. It also means pH squeezes an enormous range into a tidy span from 0 to 14.

That logarithmic nature has real consequences. Lowering a pool’s pH from 8 to 7 is not a small nudge, it is a tenfold shift in acidity, which is why pool and aquarium guides warn against big sudden adjustments.

The 0 to 14 range is a convention, not a limit. Concentrated hydrochloric acid can measure below 0 and concentrated lye above 14; the scale simply stops being the convenient way to describe them.

And 7 is not always neutral. The whole scale rests on water’s self-ionisation, and that equilibrium shifts with heat. Neutral is defined as the point where [H⁺] equals [OH⁻], which lands at exactly half of pKw, the negative log of the water ionisation constant:

So a kettle of pure boiling water reads pH 6.13 and is not acidic at all. It has exactly as many hydrogen ions as hydroxide ions, which is the actual definition of neutral. The calculator takes a temperature so that pH + pOH adds up to the right figure for the water you are measuring, rather than assuming 14 everywhere.