Reading time is worth estimate in many contexts of engineering design such as the directional sign presented on narrow space, the design of technology interface in car for user to read while driving, or process simulation of people evacuate from any disaster that need to read some word sign. While the majority of researches in this area have been focusing on the development of reading skills, where traditional study were relied on reading speed to apply on word reading time. Thus, the purpose of this study was to estimate the reading time of familiar and non-familiar words concerning individual differences in term of IQ level. A total of 59 Thai natives participated in this study. Results provided reading time estimating equations and pattern of reading time. Implications for designers are discussed.