Students who are proficient academic language (AL) users, achieve better in school. To develop students’ AL register teachers’ AL input is necessary. The goal of this study was to investigate the extent of AL features in the language input first and second grade teachers give their students in whole class mathematics instruction. Five key features could be distinguished: lexical diversity, lexical complexity, lexical specificity, syntactic complexity and textual complexity. Teachers used all features, but the amount in which they used them varied. While all teachers used lexical specific language when teaching mathematics, they did not use very complex language input. The academicness of teachers’ input was significantly higher in grade 2 than in grade 1 with respect to lexical diversity and lexical specificity. The input during explanation and discussion only differed with regard to textual complexity, which was higher during explanation.