The calculator below computes the week number of a given date by four different methods.
First - Standard way:
The week number is computed as defined in ISO 8601 - the official way to do so in Europe. According to standard, the first week of a year is a week with the first Thursday of a year, or which contains the 4th of January. With this method, 1st, 2nd, and 3rd of January can be included in the last week of the previous year, or 29th, 30th, and 31st of December can be included in the first week of next year.
Second - US way:
The first week begins on the 1st of January; next week begins next Sunday.
Third - casual way :):
The first week begins on the 1st of January; next week begins next Monday.
Fourth - simple numbering:
The first week begins on the 1st of January, the next week begins on the 8th of January, and so on.