fmol

Fmol, short for femtomole, is a unit of amount of substance equal to 10^-15 moles. It is a submultiple of the mole, used to express very small quantities of chemical or biological material. In practical terms, 1 fmol corresponds to about 6.022 × 10^8 molecules, based on Avogadro’s constant.

Fmol is commonly employed in chemistry, biochemistry, and molecular biology to quantify trace amounts of substances such as nucleic acids, peptides, enzymes, or small-molecule metabolites in assays, reaction setups, and analytical measurements. It is frequently encountered in contexts like PCR, sequencing library preparation, proteomics, and microfluidic work, where materials are scarce or where high sensitivity is required.

The mass corresponding to 1 fmol depends on the molar mass of the substance. Mass = amount × molar mass, so a 1 fmol sample of a 1-kDa molecule would weigh roughly 1 pg, while the same amount of a 1000-Da molecule would weigh about 1.0 pg; for heavier molecules the mass scales linearly with molar mass. This means fmol is a unit of quantity rather than a direct mass measure.

Note that fmol is distinct from other similarly named units such as fm (femtometer) or fs (femtosecond), which measure length and time, respectively.

See also: mole, prefixed SI units, Avogadro constant, molar mass.