Except for their wide applications in everyday life, computational estimations help students to acquire the number sense and deeper understanding of the arithmetic operations. In the present research we examined, the abilities of 50 Greek pre-service elementary teachers in computational estimation, through personal interviews. It was indicated that a large percentage of these teachers do not know how to use computational estimation and resort to exact calculation and in particular, they use written algorithms. In addition, pre-service teachers do not know how to use the strategies of averaging and compatible numbers. These pre-service teachers seem to have a limited ability to number sense, namely in mental calculations and flexibility of strategy use.