The purpose of this study is to investigate the computational estimation ability of adult learners and to implement a teaching intervention about computational estimation in an Adult Junior High School. Adults’ estimation skill is measured through interview, before and after the intervention, with a researcher-designed tool. A questionnaire is also used to evaluate their attitudes towards mathematics. The math teacher who implemented the teaching experimental is interviewed too. The results suggest that, although adults could estimate to some extent, their estimation skill was significantly improved due to the teaching intervention. Factors that may affect estimation skill, such as prior knowledge of strategies and involvement in everyday activities with measurements or mental calculations, are also investigated. Furthermore, math teacher’s participation in this research may have led to a PCK expansion, even though her attitude towards estimation was neutral. Finally, educational implications are discussed too.