With the prevalent use of visual interfaces and the increasing demand to display more information, information complexity in human–computer interfaces becomes a major concern for designers. Complex interfaces may adversely affect the effectiveness, efficiency, and even the operational safety of a system. Previously, two questionnaires were developed by researchers at the Federal Aviation Administration to evaluate information complexity of air traffic control displays. This study adapted the questionnaires for commercial visual interfaces and validated them with two types of tasks on three travel websites. The questionnaire measures the information complexity of a visual display based on perceptual, cognitive, and action complexity in terms of three complexity factors: quantity, variety, and relation. The result demonstrates that the questionnaire that uses multiple items for measuring complexity construct has good reliability, validity, and sensitivity. Information complexity is also found to be negatively correlated with usability and positively correlated with mental workload. The contribution of the study includes validating the theoretical framework for the information complexity concept through the use of questionnaires and providing a practical tool for designers to measure information complexity of the visual display for iterative improvement. © 2011 Wiley Periodicals, Inc.