The ultraviolet (UV) doses of American young adults were never measured, but are needed for assessing UV-related health risks. These doses were calculated using a novel approach. The National Human Activity Pattern Survey recorded the daily minute-by-minute activities of about 2000 young adults (0–19 years) over the course of 2 years to assess their exposure to environmental pollutants. From that survey, only the outdoor daylight data of northern and southern girls and boys were extracted and stratified by season and age to find the time American children (0–5 and 6–12 years) and adolescents (13–19 years) spend outside. They spend about 10% of the day outdoors, but only get about 30% of the available terrestrial UV radiation (on a horizontal plane). American children have about the same percent personal ambients as adults (3.1%), 2.8% for girls and 3.4% for boys. Adolescents have the lowest personal ambients (2.6%), 2.1% for girls and 3.1% for boys. To get their UV doses, their percent ambients are multiplied by the total available terrestrial UV. Excluding vacation, the erythemally weighted UV doses for American children are 25 kJ/m²/year, 23 for girls and 28 for boys. Adolescents get the lowest UV exposure of any group, 21 kJ/m²/year, 18 for girls and 24 for boys. Young adult northern girls get 18 kJ/m²/year and boys get 21 kJ/m²/year, whereas southern girls get 24 kJ/m²/year and boys get 31 kJ/m²/year. The youngest children (0–5 years) get slightly higher summer doses. Thus, we can now assess the UV-related health risks for American children and adolescents.