William Harvey's famous quantitative argument from De motu cordis (1628) about the circulation of blood explained how a small amount of blood could recirculate and nourish the entire body, upending the Galenic conception of the blood's motion. This paper argues that the quantitative argument drew on the calculative and rhetorical skills of merchants, including Harvey's own brothers. Modern translations of De motu cordis obscure the language of accountancy that Harvey himself used. Like a merchant accounting for credits and debits, intake and output, goods and moneys, Harvey treated venous and arterial blood as essentially commensurate, quantifiable and fungible. For Harvey, the circulation (and recirculation) of blood was an arithmetical necessity. The development of Harvey's circulatory model followed shifts in the epistemic value of mercantile forms of knowledge, including accounting and arithmetic, also drawing on an Aristotelian language of reciprocity and balance that Harvey shared with mercantile advisers to the royal court. This paper places Harvey's calculations in a previously underappreciated context of economic crisis, whose debates focused largely on questions of circulation.