In regard to the concept of estimation, there have been various definitions in the literature. Some of them are as follows: "process of being able to provide an adequate answer to a problem (Reys, 1986)", "quickly having an idea about the quantity or size of something without actually counting or measuring it (Micklo, 1999)". NCTM (2000) considered understanding the numbers and operations, ensuring fluency in arithmetic operations and improving the number sense as the center of mathematics education. Number sense can be defined as a person's having a good level of knowledge on the relationships of numbers and operations and the ability to use this knowledge in the situations concerning daily life (McIntosh, Reys, & Reys, 1992; Yang, 2003). In this respect, the purpose of the study is to examine the relationship between estimation skills based on calculation, estimation strategies and number sense of prospective elementary teachers. The sample of the study is comprised of prospective teachers currently studying in 1st, 2nd, 3rd, and 4th grade of classroom teaching in the Faculty of Education of a Turkish public university. The test determining the estimation skills developed by Tekinkir (2008) and test of number sense will be used as the data collection tool. The data obtained will determine the estimation skills and level of number sense of prospective elementary teachers. The study is considered to reveal what extent reasoning which is one of the fundamental skills aimed to be acquired in mathematics program in Turkey is developed among the prospective teachers and to contribute the literature. (C) 2015 Published by Elsevier Ltd.