ISO 8601 Week Numbers: How Calendar Weeks Are Counted
Why week 1 does not always start on 1 January, which years have 53 weeks and how to write week numbers correctly – with a table for the coming years.
Date & Age2 linked calculatorsReading time: 6 min.
Germany counts calendar weeks according to the international standard ISO 8601, published there as DIN ISO 8601. It states that a week starts on Monday and defines which week is the first of the year. As a result, 1 January sometimes still belongs to the last week of the previous year – and some years have 53 weeks.
Key takeaways
- A calendar week starts on Monday and ends on Sunday.
- Week 1 is the week with the first Thursday of the year – equivalently, the week that contains 4 January.
- A year has 53 weeks if it starts on a Thursday, or if it is a leap year starting on a Wednesday; otherwise 52.
- 28 December always lies in the last week of its year.
- The ISO 8601 notation is, for example, 2027-W01 for the first week of 2027.
The rule: week 1 contains the first Thursday
ISO 8601 counts weeks from Monday to Sunday. The Thursday decides which week is the first one:
- If 1 January falls on a Monday, Tuesday, Wednesday or Thursday, it belongs to week 1 of the new year.
- If 1 January falls on a Friday, Saturday or Sunday, it belongs to the last week of the previous year (week 52 or 53). Week 1 then starts on the following Monday.
- The same applies at the end of the year: the days after the last Thursday in December can already belong to week 1 of the next year.
Rule of thumb
A week belongs to the year in which its Thursday falls. Thursday is the middle of the week, so at least four of its seven days lie in that year.
What week is it today?
Week number for a date, Monday and Sunday of a week, and every week of a year.
Years with 52 and 53 weeks
A year has 365 or 366 days, which is 52 weeks plus one or two days. That is why 71 out of 400 years have a 53rd week, five to seven years apart. The table shows how many weeks the coming years have and when their week count starts and ends.
| Year | Weeks | Monday of week 1 | Sunday of the last week |
|---|---|---|---|
| 2026 | 53 | Mon, Dec 29, 2025 | Sun, Jan 3, 2027 |
| 2027 | 52 | Mon, Jan 4, 2027 | Sun, Jan 2, 2028 |
| 2028 | 52 | Mon, Jan 3, 2028 | Sun, Dec 31, 2028 |
| 2029 | 52 | Mon, Jan 1, 2029 | Sun, Dec 30, 2029 |
| 2030 | 52 | Mon, Dec 31, 2029 | Sun, Dec 29, 2030 |
| 2031 | 52 | Mon, Dec 30, 2030 | Sun, Dec 28, 2031 |
| 2032 | 53 | Mon, Dec 29, 2031 | Sun, Jan 2, 2033 |
Calculated with our calendar week calculator using the ISO 8601 rules; the same applies to the 71 years with 53 weeks in the 400-year cycle of the Gregorian calendar.
If the Monday of week 1 is still in December, the last days of December already count towards the new week-numbering year. If you use weeks for rosters, deliveries or invoices, always state the year together with the week.
Writing week numbers correctly
ISO 8601 has its own notation for weeks: the year, a capital W and the two-digit week number, for example 2027-W01. Adding a digit for the weekday (1 = Monday to 7 = Sunday) turns it into a date: 2027-W01-1 is the Monday of the first week of 2027.
- 2027-W01: first calendar week of 2027 (Monday 4 January to Sunday 10 January 2027)
- 2027-W01-5: the Friday of that week, i.e. 8 January 2027
- 2027-01-04: the same date as 2027-W01-1 in the usual year-month-day notation
- In everyday German usage you will see "KW 1/2027"; it means the same week.
When other calendars count differently
Not every calendar follows ISO 8601. Some calendars and programs start the week on Sunday or count the week containing 1 January as week 1. The numbers then differ by one week – typically around the turn of the year.
Careful with spreadsheets and calendar apps
Check which rule your software uses to count weeks. For appointments with partners abroad, a specific date is safer than a week number.
Which weekday was it?
The weekday of any date, with its week number.
Frequently asked questions
How many weeks does a year have?
Why does 1 January sometimes belong to week 52 or 53?
On which day does a calendar week start?
How do I write a week number according to the standard?
Sources
Every figure in this article comes from these primary sources or from the linked calculators, which are based on the same sources.
- ISO 8601 (date and time format), International Organization for Standardization (accessed September 30, 2026)
- Units and Time Act (EinhZeitG), section 4 (legal time) (accessed September 30, 2026)
- Physikalisch-Technische Bundesanstalt (PTB): realisation of legal time (accessed September 30, 2026)
Full-Stack Developer & Founder of HEADON.pro
Full-stack developer and founder of HEADON.pro. Developer of Rechnerzentrale.