Recently, localities across the United States have passed laws requiring the mandatory labeling of calories in all chain restaurants, including fast food restaurants. This policy is set to be implemented at the federal level. Early studies have found these policies to be at best minimally effective in altering food choice at a population level. This paper uses receipt and survey data collected from consumers outside fast food restaurants in low-income communities in New York City (NYC) (which implemented labeling) and a comparison community (which did not) to examine two fundamental assumptions necessary (though not sufficient) for calorie labeling to be effective: that consumers know how many calories they should be eating throughout the course of a day and that currently customers improperly estimate the number of calories in their fast food order. Then, we examine whether mandatory menu labeling influences either of these assumptions. We find that approximately one-third of consumers properly estimate that the number of calories an adult should consume daily. Few (8% on average) believe adults should be eating over 2,500 calories daily, and approximately one-third believe adults should eat lesser than 1,500 calories daily. Mandatory labeling in NYC did not change these findings. However, labeling did increase the number of low-income consumers who correctly estimated (within 100 calories) the number of calories in their fast food meal, from 15% before labeling in NYC increasing to 24% after labeling. Overall knowledge remains low even with labeling. Additional public policies likely need to be considered to influence obesity on a large scale.