What the ISO week number is
The ISO 8601 week number labels each week of the year from 1 to 52 (sometimes 53). Weeks start on Monday, and week 1 is the week containing the first Thursday of the year — so early January days can belong to the last week of the previous year. Businesses use it for scheduling and reporting.
Worked example: 1 January 2027 falls on a Friday, and the first Thursday of 2027 is 7 January, so that Friday belongs to week 53 of 2026 rather than week 1 of 2027. The rule works in reverse too, which is why a late December date can sit in week 1 of the following year. The pay-off is consistency: every ISO week has exactly seven days, runs Monday to Sunday and belongs to exactly one ISO year.
The days between dates calculator answers the question that usually follows this one.
People working on this usually reach for the weeks to days calculator too.
How to use this calculator
Pick a date and read its ISO week number and the year that week belongs to. Note the year shown may differ from the calendar year for dates right at the start or end of January or December.
Week numbers are the backbone of production, logistics and retail planning. "Ship in W23" or "the W40 campaign" is unambiguous across offices in different countries in a way that "early June" is not. They are standard practice in Europe and less common in the US, so it is worth naming the standard when the two meet.
Anchors for 2026: week 1 begins on Monday 29 December 2025, week 26 begins on 22 June 2026, and the year runs to week 53.
Many people follow this up with the weeks between dates calculator.
Frequently asked questions
- How do I find the week number of a date?
- Use the ISO 8601 rule: weeks start Monday and week 1 holds the year's first Thursday. This calculator computes it for any date.
- Why might early January be week 52 or 53?
- If those days fall before the year's first Thursday, ISO assigns them to the final week of the previous year.
- How many weeks are in a year?
- 52 in most years, and 53 in years where the extra days push past a full 52 weeks under the ISO rule.
- Do weeks start on Sunday or Monday?
- ISO weeks start on Monday. Some US calendars use Sunday, which can give a different week number.
- Does Excel give the same week number?
- Often not. The default WEEKNUM function starts weeks on Sunday and always puts 1 January in week 1, which can differ from ISO by one. Use ISOWEEKNUM, or WEEKNUM with return type 21, to match the result here.