Week Number Calculator

A week number is a 1–53 identifier for a week within a year, used in project planning, manufacturing, retail reporting, payroll, and academic calendars. Two systems dominate. The ISO 8601system is the international standard: weeks start on Monday and the first week of the year is the one containing January 4 (equivalently, the year's first Thursday). The US convention starts weeks on Sunday and puts January 1 in week 1 every year, no matter what day of the week it falls on. The two can disagree at the year boundary — ISO can place January 1 in week 52 or 53 of the previous ISO year, which the US system never does. This calculator returns both, plus a full-year table of every week with its Monday and Sunday dates. Pair it with the Day of Week Calculator to check the weekday a particular date falls on, or the Age Calculator for a full birth breakdown.