Week Number Calculator NZ
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.
How it works
The calculator finds the Thursday of the week containing your date, then counts how many weeks that is from the first Thursday of the year, giving the ISO week number. The ISO week year is the year of that Thursday, which is why a late-December date can show as week 1 of the following year.
Worked example
The 1st of January 2026 is a Thursday, so it falls in week 1 of 2026. The 29th of December 2025, a Monday, falls in week 1 of the 2026 ISO week year even though the calendar year is still 2025.
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