The subjective integration of numbers that are encountered sequentially is an elementary judgment process that is highly relevant in research (e.g., in a decisions from experience paradigm) and in everyday life alike (e.g., when keeping track of spending during a shopping trip). Toward a better understanding of how people perceive and integrate numerical information, participants in a laboratory experiment (n = 40) repeatedly estimated the sum of a number sequence briefly presented on a computer screen. Results indicate a systematic bias toward underestimation that could be captured with a compressive power function. The observed underestimation depended on the sequential order in which the numbers were presented but not on the shape of the underlying frequency distribution. Similar results were obtained in a field study where customers in a grocery store (n = 966) systematically underestimated the total value of their shopping basket prior to checkout. A model comparison approach revealed that the observed underestimation in the lab study was best captured by a compressed mental number line when evaluating single items, while in the field study, the bias rather stems from a systematic error during information integration. The field study further indicated that underestimation increased with age but was not due to a simple rounding strategy or the systematic forgetting of unhealthy items such as sweet or fatty snacks. The results yield novel insights into how people perceive and integrate numbers. (PsycINFO Database Record (c) 2018 APA, all rights reserved)