Finland: the overtime premium is counted by the day, not by the week
Most countries decide what your overtime is worth by looking at the week. Finland looks at the day. The Working Hours Act (872/2019) sets ordinary time at eight hours a day and 40 a week, then pays 150% for the first two overtime hours of each day and 200% for everything after that.
Finland counts from the third overtime hour of each day, so how your hours fall across the week changes the answer, and the tool says so when it applies the rule weekly. Ten overtime hours spread as two a day across five days are all at 150%. The same ten worked in a single push, say two days of five extra hours, put six of them at 200%. Same hours, same week, materially different pay. The calculator assumes the five-day spread, which is the common case and the generous-to-the-employer reading; if your hours bunch, your actual pay is higher than the figure here.
A ceiling sits above all of this, though not the one older guidance describes. The 2019 Act dropped the previous quotas of 138 overtime hours in four months and 250 in a calendar year, and replaced them with a limit on total working time: including overtime, the average may not exceed 48 hours a week over any four-month period. The premium is not a licence to work indefinitely at double time.
Annual holiday accrues at two and a half days a month once you have a year of service, which the Annual Holidays Act counts on a six-day basis and so reads as 30 days there and 25 working days here. With about eleven public holidays on top, close to a seventh of the paid year is not worked.