This research quantifies the difficulty of mathematics lessons drawn from mathematics textbooks commonly used in the United States. It also documents the mathematical complexity of textbook lessons within and across grades. Lessons were extracted from selected textbooks used in grades K-12 in the United States and analyzed. Textbooks aligned with the Common Core State Standards for Mathematics (CCSSM) and those not aligned with CCSSM were measured using the Quantile Framework for Mathematics, and each lesson was assigned a Quantile measure to represent its mathematical difficulty. The results of this research show that the median mathematical difficulty of textbook lessons consistently increases with grade, and that within grades, lessons vary in their mathematical complexity.