Interactive simulations can be engaging tools for student learning, allowing students to explore phenomena by asking questions and seeking answers through use of the sim. PhET simulations allow this process to happen dynamically so that students can continuously probe and explore the underlying science. For students to use simulations productively, understanding the science in the simulation must be challenging enough to maintain students' interest, but not so challenging that students are overwhelmed. A key aspect of achieving a good balance is the complexity of the simulation for students. We have formulated an initial model to quantify complexity based on the number, range, and effects of controls and representations within a simulation. We account for students' prior knowledge by adjusting the measured complexity depending on how students interpret the representations and conceptual connections within the simulation. Implications for simulation design and student engagement will be discussed in light of preliminary interview data.