yoctometre

A yoctometre is one septillion-th of a metre, or 10−24 m. It is the smallest length unit in the International System of Units (SI). The symbol is ym. This scale exists in the formal SI standard but has no practical application in measurement because no instrument can resolve distances this small, and few physical phenomena operate at this scale in ways that demand description.

The yoctometre belongs to a series of SI prefixes based on powers of 10. The name comes from the prefix yocto, which derives from the numeral eight in Latin (from octo), marking it as the eighth prefix below the base unit. The sequence runs: milli (10−3), micro (10−6), nano (10−9), pico (10−12), femto (10−15), atto (10−18), zepto (10−21), and finally yocto (10−24). The prefix was formally adopted in 1991.

To grasp the scale: an atom is roughly 10−10 m across. A proton is about 10−15 m. At 10−24 m, a yoctometre is far smaller than any subatomic particle. Quantum uncertainty and the Planck length (about 10−35 m) both set hard limits on what can meaningfully be measured or defined at such scales. The yoctometre exists as a unit of convenience for dimensional analysis and symbolic calculation rather than for empirical work.

Metrologists maintain yoctometre in SI tables for completeness and consistency. Its existence allows any calculation involving very small lengths to be expressed in proper SI units without resorting to scientific notation. In practice, scientists working near the quantum realm use natural units, Planck units, or angstroms and femtometres instead. The yoctometre rarely appears outside theoretical physics, mathematics, and formal standards documents.