Background: Dyspnea is a hallmark symptom of idiopathic pulmonary fibrosis (IPF), and dyspnea induced physical activity limitation is a prominent driver of quality of life impairment among IPF patients. Methods: We examined response data for the 21 physical activity items (the first 21 of 24) from the University of California San Diego Shortness of Breath Questionnaire (UCSD) collected at baseline in a recently conducted IPF trial. We used Rasch analysis and hypothesis testing with conventional statistical methodology to achieve three objectives: 1) to examine the items to identify the one characteristic that distinguishes one from another; 2) to asses these items for their ability to measure dyspnea severity in IPF; 3) to use the items to develop a dyspnea ruler. Results: The sample comprised 178 subjects. The 21 items fit the Rasch model. There was very strong correlation between Rasch item severity and their metabolic equivalents (METS) values (r = -0.86, p < 0.0001). With the sample stratified on scores from the 21 items, there were significant between group differences in FVC%, DLCO% and distance walked during the six-minute walk test. The dyspnea ruler can be used to put dyspnea levels in a more easily understood clinical context. Conclusions: The first 21 items from the UCSD compose a unidimensional dyspnea-with-activity scale and are both sensibly ordered and distinguished from each other by their METS values. These 21 items can be used confidently to formulate clinically-relevant inferences about IPF patients and should be considered for use as a meaningful endpoint in IPF research. © 2013 Elsevier Ltd. All rights reserved.