The federal rule
The Fair Labor Standards Act requires 1.5 times your regular rate for every hour over 40 in a single workweek. A workweek is any fixed seven days your employer picks. There is no federal daily overtime, no federal weekend or holiday premium, and no averaging across two weeks to dodge it: 50 hours one week and 30 the next is 10 hours of overtime.
States with daily overtime
California pays 1.5x after 8 hours in a day and 2x after 12, plus 1.5x for the first 8 hours of a seventh straight day and 2x beyond. Alaska and Nevada pay 1.5x after 8 hours in a day, and Colorado after 12. Hours already paid as daily overtime don't count again toward the weekly 40, which is what the calculator does for you. Nevada's daily rule only applies below a wage threshold, so check it if you earn well above minimum wage there.
Who doesn't get overtime
Salaried employees above the federal threshold whose duties are executive, administrative or professional are exempt. Most hourly workers are not, whatever the job title says. If your pay mixes two rates or includes a bonus, use the blended rate calculator, and see how much of it you can deduct with the no-tax-on-overtime calculator.