`1 "u" = 1.660 539 066×10^(−27) kg`

1.66053873E-27

The Atomic Mass Unit (u), also known as the “unified atomic mass unit” is not really a unit at all but a physical quantity denoted by mu and defined as 1/12 of the mass of an atom of 12C. 

The Math / Science

An atomic mass unit (amu), also called the unified atomic mass unit (u) or Dalton (Da), is a very small unit of mass used to express the mass of atoms and subatomic particles.  It is defined as:

• 1 amu = 1/12​ the mass of a carbon-12 atom (the isotope of carbon with 6 protons and 6 neutrons).
• 1 amu = 1.660 539 066×10⁻²⁷ kg

Atomic Mass units are useful because atoms and molecules are far too small to conveniently describe in kilograms or grams.  For example:

• A hydrogen atom has a mass of about 1.008 u.
• An oxygen atom has a mass of about 16 u.
• A water molecule (H₂O) is about 18 u.

Relationship to moles:

• Since 1 mole is 6.022×10²³ particles, the mass in grams of 1 mole of atoms ≈ the mass in amu of one atom.
  • Example: Oxygen ~16 amu → 1 mole of oxygen atoms ≈ 16 g.

In short, the atomic mass unit is a bridge between the microscopic world of atoms and the macroscopic world of grams and kilograms we use in the lab.