The 2010 edition of the Highway Capacity Manual (HCM 2010) provides new capacity models for single-lane and multilane roundabouts, which generally predict lower capacity than other popular models previously used in the United States. Acknowledging that both roundabout design practices and the public's use of roundabouts are still maturing in the United States, the HCM recommends local calibration of the capacity models to best reflect local behavior and specifies a calibration procedure that uses two gap acceptance parameters: the critical headway and the follow-up headway. This calibration procedure was used in a case study to develop a calibrated HCM model that was based on driver behavior at three single-lane roundabouts in Carmel, Indiana. Both gap acceptance parameters in Carmel were significantly lower than the default values in the HCM 2010. For validation, the calibrated HCM model was compared with the actual 1-min flow rates collected in the field during the congested periods. The calibrated HCM model was consistent with the field conditions for the low to medium circulating flow rates and overestimates the capacity for the high circulating flow rates. Other findings include the impact of exiting traffic, circulating flow rates, degree of saturation, and amount of the gap acceptance data on the calibration results.