Convert Span (Cloth) (span) to Femtometer (fm) instantly.
About these units
Span (Cloth) (span)
The span is a unit traditionally used in textile measurement, equal to the distance between the tip of the thumb and the tip of the little finger when the hand is fully extended, approximately 22.86 cm (9 inches). This anthropometric unit was widely used by weavers and cloth merchants to measure lengths of fabric quickly and intuitively. Its small scale made it convenient for practical applications where tape measures or rulers were unavailable. The span also appears in cultural and historical texts as a natural unit of human proportion. While largely obsolete today, it offers insight into pre-industrial textile practices and the anthropometric basis of early measurement systems.
Femtometer (fm)
A femtometer, equal to 10⁻¹⁵ meters, is the scale at which the structure of atomic nuclei becomes measurable. Also known historically as a "fermi," this unit is used extensively in nuclear physics to describe the radii of protons, neutrons, and nuclei, which typically span 1–10 femtometers. At this scale, the strong nuclear force dominates interactions, and classical intuition breaks down almost entirely—quantum mechanics provides the only meaningful framework. The femtometer also plays a role in high-energy particle experiments, where the wavelengths of probing particles (like high-velocity electrons) may be expressed in femtometer increments. These small wavelengths allow researchers to resolve sub-nuclear structures. While invisible to any optical instrument, distances in the femtometer range can be inferred through scattering experiments, such as those performed in particle accelerators.