Week Number Calculator

This calculator finds the ISO 8601 week number for any date, the standard week-of-the-year used in business, scheduling and international systems. Most of us think in dates, but a lot of the world runs on week numbers, where each week of the year is labelled from one through to fifty-two or, in some years, fifty-three. The ISO 8601 standard is the version used in commerce and across much of the globe, and it has two key rules: weeks start on Monday, and week one is the week containing the year's first Thursday, which is the same as the week containing the fourth of January. Those rules mean the week number does not always line up neatly with the calendar year, so a date in early January can fall in the last week of the previous year, and a date in late December can belong to week one of the next, which is why the calculator also shows the ISO week year. You enter a date, and it returns the ISO week number and the week year. Use it to interpret schedules and delivery dates given by week, to coordinate with overseas teams who plan by week number, or simply to answer what week of the year it is. A precise ISO week-number reference for any date.

ISO 8601: weeks start Monday and week 1 contains the year's first Thursday. The week year can differ from the calendar year near the year boundary.