This paper explores the complexity of secondary mathematics from the perspective of cognitive load theory, defining five variables that collectively measure complexity from its different sources. The variables include the number of steps required to solve a task, the quantity of elements involved in performing operations, the amount of related knowledge, the number of statement elements, and the number of required statement elements. The study uses data from N = 130 completed tasks by 81 Year-7-students. Students were asked to solve two tasks and to rate their complexity in a Paas 9-point rating scale. Results showed variables as reliable measures for complexity (α = .80), with all of them positively correlating with perceived complexity. The quantity of related knowledge and the number of elements involved in operations were proven to have the most significant effects. The five variables provide concrete, reliable, and pondered criteria for determining the complexity of early secondary mathematics, extending the perspectives currently given by cognitive load theory.