How the ISO week number is set
ISO 8601 weeks start on Monday and end on Sunday. The anchor is the Thursday: every Thursday belongs to the same ISO year, so counting Thursdays from the start of the year gives the week number without any ambiguity at the year boundary.
ISO vs US week numbering
| ISO 8601 | US | |
|---|---|---|
| Week starts | Monday | Sunday |
| Week 1 is | First with a Thursday | Week with 1 January |
| Partial weeks | None | Possible at year edges |
| Used by | Europe, business, IT | US, Excel default |
The two systems can disagree by one at the start and end of a year, so always state which one you mean when a week number goes in a schedule or report.
The year-boundary catch
- 1 January is not always week 1 — if it falls on a Friday, Saturday or Sunday it belongs to week 52 or 53 of the previous ISO year.
- Late December can be week 1 — the same rule pulls 29–31 December into the next year's week 1 when they share a week with a January Thursday.
- 53-week years — 71 of every 400 years have a week 53. The next ones are 2026, 2032 and 2037.
Common questions
What is an ISO 8601 week?
It is the international standard for numbering weeks. Weeks run Monday to Sunday, and week 1 is the first week that contains a Thursday, which is the same as the week containing 4 January. Dates are written like 2026-W19.
How many weeks are in a year?
Most years have 52 weeks, because 52 x 7 is 364 and a year is 365 or 366 days. Every five or six years an extra day builds up into a 53rd week. A year has 53 ISO weeks when it starts or ends on a Thursday; 2026 is one.
Why does Excel give a different week number?
The default WEEKNUM function uses the US system, where the week starts on Sunday and week 1 is the week containing 1 January. For the ISO number, use ISOWEEKNUM instead.
