The core principle governing the calculation of relative atomic mass utilizes Carbon-12 (¹²C) as the immutable invariant standard, where one atom is defined to possess a mass exactly equal to 1/12th of that of an unbound neutral carbon-12 atom. Relative atomic mass ($A_r$) is formally defined as the dimensionless ratio of the average mass per nucleon of an element's atoms in their ground state to this specific Carbon-12 standard unit, weighted by natural isotopic abundance. This concept resides within analytical chemistry and physical metrology, functioning as a scalar quantity that normalizes atomic masses across different laboratory scales without dependence on absolute units like grams or kilograms.
Prerequisite chain context: requires Atomic Mass Values in Chemistry.