Why the weekly threshold is not 40 hours everywhere
Forty hours is the United States federal figure, set by the Fair Labor Standards Act, and it is the number most overtime calculators quietly assume. It is not the international norm. Among the jurisdictions modelled here the weekly threshold ranges from 40 to 52 hours, and using the wrong one does not produce a slightly wrong answer, it produces a confidently wrong one.
That matters most for anyone running a team across borders, where the same 48-hour week is unremarkable in one country and overtime in another. It also matters for the reverse case: assuming a higher threshold than applies means underpaying somebody who was entitled to a premium, which is the expensive direction to be wrong in.