If you have ever been asked to deliver something "by week 34" and had to guess which days that covers, you have run into ISO 8601 week numbering. It is used constantly in European business, manufacturing scheduling, payroll and project planning — and it has two rules that surprise people the first time they meet them.
The two rules that matter
- Weeks start on Monday and end on Sunday. Monday is day 1; Sunday is day 7.
- Week 1 is the week containing the first Thursday of January.
That second rule causes the confusion, so it is worth restating in equivalent forms. Week 1 is the week containing 4 January. It is also the first week with the majority of its days — at least four — in the new year. All three statements describe the same week.
The consequence: the first days of January sometimes belong to the last week of the previous year, and the last days of December sometimes belong to week 1 of the next year.
An ISO week can span two calendar years, so a date's "ISO year" is not always its calendar year. 1 January 2027 falls on a Friday, which puts it in the final week of ISO year 2026 rather than week 1 of 2027.
Worked examples
| Date | Weekday | ISO week | Why |
|---|---|---|---|
| 1 Jan 2026 | Thursday | 2026-W01 | It is itself the first Thursday |
| 1 Jan 2027 | Friday | 2026-W53 | First Thursday of 2027 is 7 Jan |
| 31 Dec 2024 | Tuesday | 2025-W01 | Its week contains Thursday 2 Jan 2025 |
| 3 Jan 2021 | Sunday | 2020-W53 | Its week began Monday 28 Dec 2020 |
To get the ISO week for a specific date without working through the rules by hand, the week number tool gives you today's week and a full-year table.
Years with 53 weeks
Most years have 52 ISO weeks. Some have 53. The test is simple: a year has 53 weeks if it starts on a Thursday, or if it is a leap year starting on a Wednesday.
Roughly 71 years in every 400 have 53 weeks — about one in five or six. Recent and upcoming examples: 2015, 2020, 2026, 2032 and 2037. A 53-week year is not an error, but it breaks a great deal of spreadsheet work built on the assumption that a year has 52 weeks, and it is a recurring nuisance in retail and payroll, where a 53-week accounting year throws off year-on-year comparisons.
The format
ISO 8601 writes a week date as 2026-W35, and a specific day within it as 2026-W35-3 — the third day, Wednesday, of week 35 of 2026.
The week number is always two digits, so week 5 is written W05, not W5. The year component is the ISO week-numbering year, which is why 31 December 2024 is written 2025-W01-2 rather than 2024-anything.
Where ISO weeks differ from what you might expect
Not every week-numbering system in use follows ISO 8601, which is why two tools can disagree about the same date.
- The North American convention commonly treats weeks as starting on Sunday and counts week 1 as the week containing 1 January, regardless of how few days of it fall in the new year. This can differ from ISO by one week number.
- Middle Eastern business calendars often start the week on Saturday or Sunday.
- Spreadsheet functions vary. Excel's WEEKNUM defaults to a Sunday-start, 1-January system; its ISOWEEKNUM function is the one that follows the standard.
- Broadcast and retail calendars — 4-4-5 accounting periods, for example — use their own week structures entirely and should not be assumed to align.
If a deadline arrives as a week number from someone in another country or another department, it is worth confirming which system they mean before assuming.
Who uses week numbers, and why
Week numbering is far more common in northern Europe than in North America. In Germany, the Nordic countries and the Netherlands it appears routinely in ordinary business conversation — a delivery date given as "KW 34" or "vecka 34" needs no further explanation.
The appeal is that weeks are a more useful planning unit than months for anything with a production or shipping rhythm. Weeks are all exactly the same length, they always start on the same weekday, and they avoid the 28-to-31-day irregularity that makes month-over-month comparison awkward. Manufacturing schedules, construction programmes and academic terms are frequently planned in weeks for exactly this reason.
They also show up in places you might not expect. Many electronic components carry a four-digit date code giving the year and week of manufacture, and some equipment serial numbers encode the same.
Working out a week number by hand
- Find the Thursday of the week containing your date — that is, the Thursday within the same Monday-to-Sunday week.
- The year of that Thursday is the ISO week-numbering year.
- Count how many Thursdays have occurred in that year up to and including this one. That count is the week number.
The Thursday shortcut works because Thursday is the midpoint of a Monday-start week, so it always falls in the year that owns the majority of the week's days.