In the context of rotational moulding (RM), linear medium density polyethylene (LMDPE) is by far the most popular thermoplastic for load-bearing parts (e.g. underground water tanks), primarily because of its wide processing window, ideal melt flow properties, low cost and excellent toughness. On the other hand, the stiffness of LMDPE is relatively low in comparison to alternative polymers such as polypropylene; this low stiffness warrants the application of finite element analysis (FEA) on load-bearing RM designs to validate their structural integrity before production. Due to the complex non-linear behaviour of PE (even at very low strains), the Poisson’s ratio is a challenging parameter to measure within the elastic region. Consequently, Poisson’s ratio tests require expensive specialist equipment such as laser/video extensometry or ultrasound techniques. Due to the latter requirements, literature regarding the Poisson’s ratio of PE and its effect on the outcome of FEA is scarcely available. In order to address this gap in knowledge, Poisson’s ratio sensitivity studies were conducted using FEA software and the results discussed.