Aim This study aimed to identify determinants of physical, mental and emotional nursing workloads. Background Workload has a physical, mental and emotional dimension. It influences employees' well‐being and quality of care. Nevertheless, studies of specific predictors for each dimension of nurses' workload are scarce. Methods We used a cross‐sectional prospective design based on the Job Demand‐Resources theory. We asked nurses to describe workload perceived at the end of every shift over three consecutive weeks. Data were gathered from two academic hospitals, in seven medical–surgical wards. We received 259 responses and tested 2 multivariate regression models. Results Physical workload was predicted from all variables tested; mental workload was determined by patient complexity or isolation, adequacy of nurse staffing and skill‐mix, and unscheduled activities; and emotional workload was predicted by all variables except adequacy of staffing and other people's education. Conclusions Patient, nurse and workflow aspects influenced nurse's shift workload differently for each specific dimension. Implications for Nursing Management Measurement and definition of predictors of workload in the work environment are essential. Recognizing the determinants of specific dimensions of workload facilitates identification of the most appropriate interventions to improve nurses' well‐being in health care settings.