Although the scientific origins of the calorie date back to the 1820s, calorie counting for weight loss only became popular in the late 1910s and 1920s. Placing this development in the broader context of the Progressive Era, this article considers how calorie counting and the reconstitution of food as calories reflected the period's fixation with science, rationalization, and quantification. This article also situates calorie counting within shifting bodily ideals among white women in the 1920s, and the ways in which class and race informed the promotion of the slender body as the feminine ideal. The second half of this article focuses on exchanges between Lulu Hunt Peters, a syndicated newspaper columnist and the author of a best-selling calorie-counting guide, and advice-seeking readers of her column. While Peters presented calorie counting as empowering for dieters and a way for them to seize control over their weight, her calorie-restriction program facilitated a new form of bodily discipline and self-regulation during a period that saw enhanced forms of surveillance in other areas of life.