What does the pH scale measure?

pH measures the acidity or alkalinity of a water-based solution on a scale of 0 to 14. It is defined as the negative base-10 logarithm of the hydrogen-ion concentration, pH = -log10[H+]. Lower values are more acidic, higher values more basic.

At 25 degrees Celsius pure water self-ionises so that hydrogen and hydroxide ions are equal at 1e-7 mol/L each, giving pH 7. The neutral point shifts slightly with temperature, so very hot water can have a neutral pH below 7.

How much stronger is one pH unit?

The scale is logarithmic, so each whole unit represents a tenfold change in hydrogen-ion concentration. A solution at pH 3 has ten times more hydrogen ions than pH 4 and one hundred times more than pH 5.

How is pOH related to pH?

In water at 25 degrees Celsius pH and pOH always add up to 14, because the product of the hydrogen-ion and hydroxide-ion concentrations is fixed at 1e-14. So pOH = 14 minus pH.

Are the listed pH values exact?

No. They are typical approximate values. Real pH varies with temperature, concentration, dissolved gases and the specific sample, so treat the table as a guide rather than a precise measurement.

pH Scale Reference Table

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A quick map of the acidity scale

The pH scale runs from strongly acidic battery acid near 0 through neutral water at 7 to caustic lye near 14. This reference lists approximate pH values for common household and natural substances, sorted from most acidic to most basic, alongside a calculator that converts a pH value into its pOH and its hydrogen- and hydroxide-ion concentrations.

How it works

pH is the negative base-10 logarithm of the hydrogen-ion concentration:

pH  = -log10[H+]
[H+] = 10^(-pH)   mol/L
pOH = 14 - pH      (in water at 25 C)
[OH-] = 10^(-pOH)  mol/L

Because the relationship is logarithmic, the difference between two pH values is a power of ten in ion concentration. Moving from pH 5 to pH 2 is a 1,000-fold increase in acidity, not a threefold one. The calculator clamps input to the 0–14 range and reports both ion concentrations in scientific notation.

Tips and notes

Acidic = below 7, neutral = 7, basic/alkaline = above 7.
Each whole pH unit is a 10× change in [H+].
Temperature shifts the neutral point: neutral water above 25 °C reads below 7.
Listed values are approximate and depend on concentration and sample source.
pH and pOH always sum to 14 in dilute aqueous solution at 25 °C.