Imprecise numerical expressions, such as “about 100 meters”, are pervasive in natural language. Mobile robotics, Geographic Information Systems, intelligent personal assistants as well as database querying applications are required to automatically and accurately interpret such expressions, called Approximate Numerical Expressions (ANE). The main challenge is to determine their numerical boundaries that sound plausible to users. The aim of this paper is to provide guidelines to interpret ANEs that are independent from the domain and the formal representations. We identified three arithmetical properties and examined their involvement in ANE interpretation as intervals of denoted values. The implicit assumption of symmetry of the intervals was also tested. To do so, 146 participants were asked to provide the intervals corresponding to 24 ANEs in a semantically neutral context. Results suggest that the properties of ANEs we identified are key factors in their interpretation while symmetry is not always maintained. This study contributes towards an understanding of how users process ANEs and its results can be used to improve intelligent interfaces that lead to better users’ satisfaction and natural interaction between him/her and the system.