Background Health system performance depends on the availability, accessibility, acceptability, and quality of health workforces. Policymakers seek whether the number of nurses is optimally matched based on patients’ needs. This study aimed to assess the workforce stock, workload activities, activity standards, and workload pressure to determine the number of required nurses in different types of hospitals in Iran. Methods This study applied the workload indicators of staffing needs (WISNs) method and was conducted in 22 surgical and internal medicine wards at five hospitals in the southwest of Iran during six months. A time-motion study, and several group discussions, interviews were used to extract the required data. Descriptive statistics were used for data analysis. Results All selected hospitals faced nursing shortages. The highest shortage (-47) and workload pressure (WISN ratio 0.45) were observed in the general-educational hospitals. In the specialized hospitals, the workload pressure was high (WISN ratio 0.49). The lowest shortage belonged to the private hospital. Based on our assessment, in all of the hospitals, nurses typically worked overtime due to high workload. The studied hospitals covered an average of 25% of their shortage with nursing overtime working. We noted that nurses were predominantly occupied with health service and supportive activities (≈90% of their time). Conclusions Based on the WISN method, all of the hospitals faced nursing shortages from moderate to high. However, it would be essential to consider current labor market analysis based on accurate data to adopt appropriate policies in HRH planning.