quadrillionth

A quadrillionth is the reciprocal of a quadrillion. In the standard short scale used in most English-speaking countries today, a quadrillion equals 10^15, so a quadrillionth equals 10^-15. In the former long scale used in some regions, a quadrillion equals 10^24, making a quadrillionth 10^-24. Because many places have shifted to the short scale, the short-scale definition is the common reference for the term.

In terms of SI prefixes, 10^-15 corresponds to the prefix femto-. Therefore, a quadrillionth of a meter is a femtometer (fm), and a quadrillionth of a second is a femtosecond (fs). This relationship shows how the colloquial term aligns with standardized scientific notation and measurements.

Usage of the word quadrillionth is relatively rare outside informal descriptions. In scientific and technical contexts, it is more common to use scientific notation (such as 1e-15) or the appropriate SI prefix (femto-). The term mainly serves a descriptive purpose when discussing extremely small fractions or durations without using specialized notation.