Precision in building was pursued and achieved well before the rise of modern science and technology. This fact applies to the classical tradition as well as to medieval architecture, and is particularly evident in architectural drawings and design from the Italian Renaissance onward. In this essay, I trace the shift from geometry-the primary tool for quantification in classical architecture- to numeracy that characterizes Renaissance architectural theory and practice. I also address some more general aspects of the relation between technologies of quantification and the making of architectural forms.