% Yield = (Actual Yield / Theoretical Yield) × 100
Percent yield measures reaction efficiency: how much product was actually obtained versus the theoretical maximum predicted by stoichiometry. % Yield = (actual/theoretical) × 100. Typical lab yields: organic synthesis 60-90%, industrial processes 85-99%. Yields below 100% result from: incomplete reactions (equilibrium), side reactions, transfer losses, impure reagents, and purification losses. Yields above 100% indicate impurities in the product. Theoretical yield is calculated from the limiting reagent using mole ratios. In multi-step synthesis, the overall yield = product of individual step yields — a 5-step synthesis with 80% each step gives only 0.8⁵ = 33% overall, emphasizing why high individual yields matter.