The importance of commercial arithmetic and double-entry bookkeeping (or “debitor and creditor” accounting) has been traced in The Merchant of Venice, Othello, the Sonnets, and other works of Shakespeare and his contemporaries. But even though both are explicitly cited in Cymbeline (the only Shakespeare play other than Othello to invoke double-entry by its contemporary English name), their importance for this late Shakespearean tragicomic romance has yet to be explored. This article traces multiple ways in which Cymbeline is impacted by arithmetic and the arts of calculation, risk-taking, surveying, and measuring; its pervasive language of credit, usury, gambling, and debt, as well as slander infidelity and accounting counterfeiting; the contemporary conflation of the female “O” with arithmetic’s zero or “cipher” in relation to alleged infidelity; and the larger problem of trust (from credere and credo) that is crucial to this play as well as to early modern England’s culture of credit.