Peak inspiratory flow (PIF) has been proposed as a measure to assess a patient’s ability to use dry powder inhalers (DPIs). However, robust quality criteria to determine a repeatability limit for measuring PIF are lacking. What are the repeatability limits for measuring PIF? What is the relationship between PIF measured using the In-Check DIAL device at Diskus (GlaxoSmithKline; PIFD) and HandiHaler (Boehringer Ingelheim; PIFHH) resistances? Data from a randomized, controlled, phase 3 trial (study 0149; see Clinical Trial Registration data) were used to define repeatability limits for PIF. In addition, a model to characterize the relationship between PIF measured with the In-Check DIAL device at PIFD and PIFHH was defined using data from two randomized, controlled, phase 3 trials (studies 0128 and 0149). In study 0128, the mean values (SD) for PIF at zero resistance and PIFHH were 84.6 (33.4) and 57.3 (26.1) L/min, respectively. In study 0149, the mean values (SD) for PIFD and PIFHH were 42.4 (11.2) and 29.0 (8.3) L/min, respectively. At the mean level, the mean difference between measurement attempts for PIFD and PIFHH was small, < 5 and < 3 L/min, respectively. The repeatability limit was determined as 10 and 5 L/min for PIFD and PIFHH, respectively. Modeling the relationship between PIFD and PIFHH, after controlling for significant covariates, demonstrated that a PIFD value of 60 L/min was approximately equivalent to PIFHH of 40 L/min. This analysis demonstrated that the two highest values of PIF using the In-Check DIAL device among three inspiratory efforts, met the repeatability limit. Altogether, these data provide guidance for measuring PIF against the simulated resistance of a specific DPI in clinical practice and research studies. ClinicalTrials.gov; Nos.: NCT02518139 (study 0128) and NCT03095456 (study 0149); URL: www.clinicaltrials.gov.