To calculate the week number in a view, enter following formula in a calculated column:
Replace the "Yourdatefieldhere" with the column that contains the date field you want to calculate the formula on. Thanks to http://lamahashim.blogspot.com/2009/10/sharepoint-calculated-field-week-number.html for the formula.
Swedish version
=HELTAL((Yourdatefieldhere-DATUM(ÅR(Yourdatefieldhere);1;1)+(TEXT(VECKODAG(DATUM(ÅR(Yourdatefieldhere);1;1));"d")))/7)+1
English version
=INT((Yourdatefieldhere-DATE(YEAR(Yourdatefieldhere),1,1)+(TEXT(WEEKDAY(DATE(YEAR(Yourdatefieldhere),1,1)),"d")))/7)+1
More examples of view formulas here