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