Week number calculator
The ISO week number, and the year it belongs to — which is not always the year the date is in. That is the part every other week calculator leaves out. Nothing you type here leaves your browser — it is worked out on your own device, so there is nothing to send and nothing to log.
The week number belongs to a different year from the date
This is the whole reason the page exists, and almost nothing on the internet says it. 1 January 2021 is in week 53 of 2020. Not week 53 of 2021 — there is no such week. And 29 December 2025 is in week 1 of 2026.
ISO 8601 defines an ISO year that is a whole number of weeks, so it has to start and end on a Monday, so it cannot line up with January the first. A report that groups rows by week number and calendar year therefore duplicates one week and loses another, every single year, and the rows involved are the ones nobody looks at because it is Christmas.
The rule, which is simpler than it sounds
Week 1 is the week containing the first Thursday of January. Equivalently, the week containing 4 January; equivalently, the first week with at least four days in the new year. Weeks run Monday to Sunday, and Monday is day 1 — not Sunday, which is where most American software starts.
Everything follows from that. The week a date sits in is decided by its Thursday, and the ISO year is whichever calendar year that Thursday lands in. Nothing else needs remembering.
Some years have 53 weeks
A year has 53 ISO weeks when it starts on a Thursday, or when it is a leap year starting on a Wednesday — about every five or six years, in an irregular pattern. 2020 had 53. 2026 has 53. Anything that assumes 52 breaks in those years, and quietly: a chart with 52 slots simply drops the last one.
The other week numberings
The ISO week is not the only one in use, which is another reason two systems disagree:
| ISO 8601 | Weeks start Monday. Week 1 contains the first Thursday. Used in Europe and in most standards. |
|---|---|
| North American | Weeks start Sunday. Week 1 contains 1 January, so week 1 can be one day long. |
| Broadcast | Weeks start Monday, week 1 contains 1 January. Used for television ratings. |
| Retail (4-5-4) | Starts in late January or early February and has its own 53-week years. Nothing to do with any of the above. |
This page does ISO 8601 and says so. If a
spreadsheet disagrees with it, check which one the spreadsheet is
doing — Excel's WEEKNUM is the North American one
by default and ISOWEEKNUM is this one.
What it will not do
- It will not do the North American or broadcast numbering. Two numberings on one page, both called “week 1”, would cause more confusion than it solved.
- It will not do retail 4-5-4 calendars, which are a company decision rather than a standard.
- It will not tell you which week your payroll thinks it is.