Objectives The objective was to quantify the correlation between general numeracy and health literacy in an emergency department (ED) setting. Methods This was a prospective cross-sectional convenience sample study of adult patients in an urban, academic ED with 97,000 annual visits. General numeracy was evaluated using four validated questions and health literacy using three commonly used validated screening tools (Short Test of Functional Health Literacy in Adults [S-TOFHLA], Rapid Estimate of Adult Literacy in Medicine-Revised [REALM-R], and the Newest Vital Sign [NVS]). Scores were dichotomized for health literacy tests to limited (low or marginal) versus adequate health literacy, and the proportion of patients answering all numeracy questions correctly was calculated with the mean proportion of correct responses in these groups. The correlation between numeracy scores and scores on the health literacy screening tools was evaluated using Spearman's correlation. Results A total of 446 patients were enrolled. Performance on questions evaluating general numeracy was universally poor. Only 18 patients (4%) answered all numeracy questions correctly, 88 patients (20%) answered zero questions correctly, and overall the median number of correct answers was one (interquartile range [IQR] = 1 to 2). Among patients with limited health literacy (LHL) by any of the three screening tools used, the mean number of correct numeracy answers was approximately half that of patients with adequate health literacy. However, even among those with adequate health literacy, the average number of correct answers to numeracy questions ranged from 1.6 to 2.4 depending on the screening test used. When dichotomized into those who answered ≤50% versus >50% of numeracy questions correctly, there was a significant difference between those with LHL and those who scored ≤50% on numeracy. Health literacy screening results were correlated with general numeracy in the low to moderate range: S-TOFHLA rs = 0.428 (p < 0.0001); REALM, rs = 0.400 (p < 0.0001); and NVS, rs = 0.498 (p < 0.0001). Conclusions Correlations between measures of general numeracy and measures of health literacy are in the low to moderate range. Performance on numeracy testing was nearly universally poor, even among patients performing well on health literacy screens, with a substantial proportion of the latter patients unable to answer half of the numeracy items correctly. Insofar as numeracy is considered a subset of health literacy, these results suggest that commonly used health literacy screening tools in ED-based studies inadequately evaluate and overestimate numeracy. This suggests the potential need for separate numeracy screening when these skills are important for health outcomes of interest. Providers should be sensitive to potential numeracy deficits among those who may otherwise have normal health literacy.