AE = (MW of desired product / MW of all products) × 100
Atom economy, introduced by Barry Trost in 1991, measures how efficiently a reaction uses its atoms. Unlike percent yield (which measures how much product you actually get), atom economy measures the theoretical maximum efficiency of the reaction itself. Addition reactions have 100% atom economy (all atoms incorporated). Substitution reactions have lower AE (leaving groups become waste). Rearrangement reactions also have 100% AE. The pharmaceutical industry averages only 20-40% atom economy — meaning 60-80% of reactant atoms become waste. Green chemistry principles prioritize high atom economy reactions to minimize waste. The E-factor (kg waste / kg product) is a related metric: pharma E-factors are 25-100+, while petrochemicals achieve <1.