National Income and Product Accounts (NIPA) measure real output growth by means of a Fisher ideal chain index. Bridging modern macroeconomics and the economic theory of index numbers, this paper shows that output growth as mea-sured by NIPA is welfare based. In a dynamic general equilibrium model with gen-eral recursive preferences and technology, welfare depends on present and future consumption. Indeed, the associated Bellman equation provides a representation of preferences in the domain of current consumption and current investment. Ap-plying standard index number theory to this representation of preferences shows that the Fisher-Shell true quantity index is equal to the Divisia index, in turn well approximated by the Fisher ideal index used in NIPA.