This study investigates pre-service elementary teachers' mathematical knowledge in teaching for length measurement. More specifically, this study compares the foundation and transformation knowledge of pre-service elementary teachers from Turkey and the United States. The current study examines similarities and differences between them by applying the codes of the Knowledge Quartet model with 40 pre-service teachers selected from Turkey and the United States. This study used a test assessing pre-service teachers' mathematical knowledge in teaching about the students' mistakes, difficulties, misconceptions, the reasons for them, and their strategies to overcome them related to the topic of length measurement. The findings indicated that while US pre-service teachers had more secure foundation knowledge in identifying the possible errors, Turkish pre-service teachers showed greater foundation knowledge in identifying the possible reasons for these errors. On the other hand, the pre-service teachers in both countries have similar transformation knowledge to suggest strategies to correct students' errors. These findings might provide some insights about how pre-service teachers' mathematical knowledge may be similar or different in different countries. Furthermore, these similarities and differences might help researchers and teacher educators emphasize the importance of the quality of teacher education programs.