The 41st hour is overtime — in any workweek
Under the US Fair Labor Standards Act, non-exempt hourly employees earn at least 1.5× their regular rate for every hour past 40 in a workweek. The test is per workweek, never per pay period: a biweekly period of 45 hours then 35 hours is 80 hours total but still carries 5 overtime hours. That is why the calculator asks for your week and lets you set the line — 40 is the federal rule; some states add daily overtime (California after 8 hours in a day), and some contracts pay double time past a second threshold.