Objectives: To study perceptions of physicians and nursing staff about real-time demands and capacity of an emergency department (ED). To use ED data to calculate proposed demand ratios called Real-time Emergency Analysis of Demand Indicators (READI) scores. To compare the READI scores with ED staff perceptions of demand and capacity. Methods: This prospective study used a computerized clinical management system to provide data about ED demand and capacity. Physicians and staff charge nurses were surveyed about perceptions of ED demand and capacity. Results were compared with mathematical READI scores, which are proposed to objectively assess ED demand. Kappa scores were used to measure intrarater reliability between the physicians' and charge nurses' assessment of demand and between the staff assessments and the READI scores. Results: Kappa scores of the perception of excess demand were as follows: between two physician groups, κ= 0.392; between one physician group and charge nurses, κ= 0.453; and between a second physician group and charge nurses, κ= 0.243. Comparing respondents who indicated that demand had or had not exceeded capacity, one of the READI ratios, the Bed Ratio, showed a significant difference in mean, 0.245 (95% confidence interval = 0.153 to 0.336), between groups. Conclusions: Real-time data may be used to predict ED demand and resource needs. Subjective assessment of excess ED demand did not correlate between physician groups or between physicians and charge nurses. Although there was a trend toward predicting excess demand with one of the READI scores, these scores did not correlate to staff perceptions.