This article explores three attributes of teachers' understanding of fraction magnitude: the accuracy and reasonableness of teachers' estimations in response to fraction arithmetic tasks as well as the alignment of the estimation strategies they used with the concept of fraction magnitude. The data were collected from a national sample of mathematics teachers in grades 3-7 in which fraction concepts were taught (N = 603). The results indicated the teachers' estimations were only partially accurate and reasonable, particularly when fraction division was involved. Furthermore, teachers' credentials and the grade level at which they taught mathematics were significantly related to teachers' understanding of fraction magnitude.