One of the work practices frequently taught to employees is to estimate the heaviness of load before it is actually handled. If it is 'heavy', then one should ask for help. However, limited information can be found in the ergonomics literature about what a person perceives as a 'heavy load'. This study was conducted on 20 male and 20 female workers in the package delivery industry to estimate the amounts of load that correspond to various levels of load heaviness (e.g. 'somewhat heavy'). Experienced employees were used for this purpose. The distribution of loads within each heaviness level was developed using fuzzy sets theory. The maximum load (i.e. 23 kg) defined by the US National Institute for Occupational Safety and Health represents a 'somewhat heavy' load based on the analysis of load distribution (corresponding to a 1.0 certainty factor). Also, the 40 kg considered in the 1981 NIOSH guidelines may be classified as a 'very heavy' load. A comparative analysis of the results of this study with norms established in prior research indicates that one should be more careful in the interpretation of statistical norms for human perception of load handling. A 'moderate' level of load heaviness (i.e. 14 kg) can be handled by 85% of the worker population.