Obesity has been shown to increase the risk of some types of injury in crashes. One hypothesis is that obesity adversely effects belt fit by changing the routing of the belt relative to the underlying skeletal structures. To evaluate this hypothesis, belt fit was measured in a laboratory study of 54 men and women, 48 percent of whom were obese, with a body mass index (BMI) of 30 kg/m² or greater. Test conditions included a wide range of upper and lower belt anchorage locations and ranges of seat height, seat cushion angle, and seat back angle spanning a large fraction of current vehicle front and rear seats. In some conditions, foot position was restricted to simulate the typical situation in the second row of a small sedan. Across individuals, an increase in BMI of 10-kg/m² was associated with a lap belt positioned 43 mm further forward and 21 mm higher relative to the anterior-superior iliac spines of the pelvis. Each 10-kg/m² increase in BMI was associated with an increase in lap belt webbing length of 130 mm. The worsening of lap belt fit with restricted foot position was slightly greater for obese participants. Obesity was associated with a more-inboard shoulder belt routing across a wide range of upper belt anchorage locations, and the shoulder belt webbing length between the D-ring and latchplate increased by an average of 60 mm with each 10-kg/m² increase in BMI. The results suggest that obesity effectively introduces slack in the seat belt system by routing the belt further away from the skeleton. Particularly in frontal crashes, but also in rollovers and other scenarios, this slack will result in increased excursions and an increased likelihood and severity of contacts with the interior. The higher routing of the lap belt with respect to the pelvis also increases the likelihood of submarining in frontal crashes.