percent
ConcentrationDimensionlessexact by definition
The percent is one part in a hundred and serves as the base unit of this dimensionless concentration type, so its factor of 1 is exact. It is the most common way to express a concentration outside the laboratory, but it is also the most ambiguous, because a percentage can be by mass, by volume, or mass per volume.
Watch out: State the basis. In water and chemical work, % w/w is grams per 100 g of solution, % v/v is millilitres per 100 mL, and % w/v is grams per 100 mL. A 12.5 percent sodium hypochlorite solution is quoted by trade convention as available chlorine by weight, while a 50 percent glycol mix is almost always by volume. The three bases can differ by tens of percent for the same physical solution.
| 1 % (Concentration) | 1 % |
| 1 % (Dimensionless) | 0.01 — |
Percent is a dimensionless ratio expressed in hundredths, so 1 percent is exactly 0.01 as a plain ratio. It is a way of writing a number rather than a unit, which is why the conversion factor is exact by definition. The related forms are per mille at one thousandth, and parts per million at 1e-6.