This article contributes in three ways to the prior literature on the reasonable doubt standard. First, it synthesizes the insular strands of historical, economic, jurisprudential, and doctrinal scholarship on reasonable doubt. Second, it advances a conception of the criminal standard of proof designed to avoid the various problems affecting earlier attempts to devise meaningful definitions of reasonable doubt. The definition proposed is that “reasonable doubt” be the standard of proof which minimizes the aggregate subjective expected social cost of false conviction and false acquittal. Judicial pronouncements of Blackstonian ratios (for example, that it is better that ten guilty go free than one innocent be convicted) are interpreted as judicial estimates of these variables, from which efficient reasonable doubt standards may be calculated. It is urged that courts adopt the precise numerical measures of certainty in jury instructions (for example, that a juror should only vote to convict if he is more than x% certain of the defendant’s guilt). Judicial pronouncements of Blackstonian ratios are collected from the caselaw of all fifty states and federal courts to encourage practitioners to test the refined conception in their jurisdiction.