When a product is described as sustainable, it is often used qualitatively to suggest it has a lower environmental impact than other products. However, these assessments lack regional or local context. Using washing machines and water use as an example, common sustainability assessments would not consider the regional availability of water or competing water demands such as for essential food growth. Considering these aspects is important if we are to determine how much of a specific activity or product/service use is permissible without compromising local, regional, or even global safe operating limits. An assessment approach that considers the efficiency and purpose of a product/service is therefore needed. Based on the approach presented in this paper, in the UK, a typical washing machine is sustainable with respect to water use given water availability, but its carbon emissions are unsustainable given the country's net positive carbon footprint. This research can be used to communicate the environmental sustainability of products in a definitive way, avoiding subjective or misleading claims, and to enable comparisons between products.