Convert a monthly salary to an hourly rate and back, from the hours worked.
Divide the monthly salary by that month's normal working hours, not by a fixed number. A salary of 1,200 euro in a month with 168 working hours gives 7.14 an hour, while the same salary in a 160-hour month gives 7.50.
Because months hold different numbers of working days and public holidays fall unevenly. On a forty-hour week a month runs from 152 to 184 hours. That is why an hourly rate worked out from a single average is wrong in particular months.
The Labour Law requires a supplement of at least one hundred percent of the set hourly rate, so an overtime hour pays double. That is why a correct hourly rate matters even on a monthly salary: it is what each overtime hour is priced from.
Contracts and time records always use the gross rate, because overtime, supplements and holiday pay are all calculated from it. A net hourly rate is useful only for personal comparison, and it differs from person to person with their tax reliefs.