Attogram

An attogram (ag) is a unit of mass in the International System of Units (SI). It represents 10^-18 grams, making it one of the smaller named mass units in common use. Because the gram is derived from the kilogram, 1 ag equals 10^-21 kilograms.

Attograms are used for describing extremely small masses, such as atoms, molecules, or nanoscale particles, where conventional units like grams or milligrams are impractical. In practice, many scientists also use the dalton (Da) or unified atomic mass unit, noting that 1 Da is about 1.66×10^-24 g, or roughly 1.66×10^-6 ag.

Examples: a hydrogen atom has a mass of about 1.7×10^-6 ag, and a water molecule about 3×10^-5 ag (roughly 3×10^-23 g). These values illustrate that attograms sit between zeptograms (1×10^-21 g) and femtograms (1×10^-15 g) on the mass scale.

Attograms are rarely encountered outside high-precision laboratory work, but they appear in nanotechnology, mass spectrometry, and biophysics when quantifying masses at the molecular or submolecular level. The term reflects SI prefixing and the continuing range of accessible masses in scientific measurement.