The aim of this study was to explore in-service teachers’ performance and strategies in computational estimation and possible individual difference. Additionally, the study looked for possible individual difference in terms of age and work experience, their attitude towards mathematics and their prior performance in mathematics during high school years. Eighty Greek in-service teachers participated in the study. Results showed that in-service teachers performed quite well, in most of the computational estimations and used a variety of strategies. Individual differences were captured only in terms of in-service attitude towards mathematics.