The ISO 8601 rules for week numbers
The international standard for dates, ISO 8601, numbers the weeks of a year with three rules:
- Every week runs from Monday to Sunday, and the weekdays are numbered 1 (Monday) to 7 (Sunday).
- Week 1 is the week that contains the year’s first Thursday. Equivalently, it is the week that contains January 4.
- Each week belongs entirely to one week-year: the year in which its Thursday falls.
The third rule is what surprises people. Because a week is never split between two years, the last days of December can belong to week 1 of the following year, and the first days of January can belong to week 52 or 53 of the previous year. The full ISO form of a date written this way is year, week and weekday, for example 2026-W42-2 for Tuesday, October 13, 2026.
Formula
A year has 53 ISO weeks when January 1 is a Thursday, or when it is a leap year that starts on a Wednesday. Every other year has 52.
Worked examples
- Tuesday is weekday 2, so the Thursday of its week is 2 days later: October 15, 2026.
- That Thursday is in 2026, so the week-year is 2026.
- October 13 is day 286 of the year, and 2026 has 53 ISO weeks because it began on a Thursday.
- The Thursday of that week is January 2, 2025.
- Its week-year is therefore 2025, and as the first Thursday of 2025 it marks week 1.
- The week runs from Monday, December 30, 2024 to Sunday, January 5, 2025.
- The Thursday of that week is December 31, 2026.
- The week-year is 2026, and since 2026 has 53 weeks this is its last one.
- Week 1 of 2027 starts on Monday, January 4, as in the calendar above.
- Week 1 of 2027 starts on Monday, January 4.
- Week 10 starts nine weeks later: January 4 + 63 days = March 8.
- It ends six days after that, on Sunday, March 14.
Where week 1 starts, 2024 to 2030
| Year | January 1 falls on | Week 1 starts | ISO weeks |
|---|---|---|---|
| 2024 | Monday | Monday, January 1, 2024 | 52 |
| 2025 | Wednesday | Monday, December 30, 2024 | 52 |
| 2026 | Thursday | Monday, December 29, 2025 | 53 |
| 2027 | Friday | Monday, January 4, 2027 | 52 |
| 2028 | Saturday | Monday, January 3, 2028 | 52 |
| 2029 | Monday | Monday, January 1, 2029 | 52 |
| 2030 | Tuesday | Monday, December 31, 2029 | 52 |
Other week-numbering systems
ISO weeks are the international standard, but they are not the only system in use. Another convention, the default in Excel’s WEEKNUM function and common in the United States, starts weeks on Sunday and calls the week containing January 1 week 1, however few days of it fall in January. The two systems often agree, then drift apart:
- Sunday, January 4, 2026 is in ISO week 1 (the last day of that week) but US-style week 2 (its first day).
- In 2027, January 1 falls on a Friday, so US-style week 1 is January 1 and 2. From then on the US count runs one ahead from Monday to Saturday and two ahead on Sundays: Wednesday, October 13, 2027 is ISO week 41 but US-style week 42.
Excel offers both: WEEKNUM with return type 21, or the separate ISOWEEKNUM function, gives the ISO number. Check which one a shared file uses.
Mistakes with week numbers
- Labeling the week with the calendar year. December 30, 2024 is in 2025-W01. Writing “2024 week 1” points to a week a full year earlier.
- Assuming every year has 52 weeks. 2026 has 53, so plans that repeat “every 52 weeks” slip by a week in years like it.
- Dividing the day of the year by 7. Day 286 ÷ 7 gives about 40.9, which suggests week 41, but October 13, 2026 is in week 42 because week 1 began on December 29, 2025.
- Mixing systems between offices. A team using ISO weeks and a partner using Sunday-start weeks can be a week apart on the same date. Quote the dates of the week, not just its number.
The day of the year counts from January 1 as day 1, so it ends at 365, or 366 in a leap year. It always uses the calendar year, even when the ISO week-year is different.
Sources
- RFC 3339: Date and Time on the Internet: Timestamps (Appendix A, ISO 8601 collected ABNF), Internet Engineering Task Force.
- WEEKNUM function, Microsoft Support.