Federal law only requires overtime past 40 hours a week. A few states add daily rules: California pays 1.5x past 8 hours/day and 2x past 12; Alaska and Nevada have daily thresholds too.
Double-time is mostly a state or contract feature, not a federal one — so two workers with identical hours can be owed very different premiums by location.
Enter your state to see which threshold applies; the weekly-only assumption undercounts pay in daily-overtime states.