ISO Week Numbers Explained: How "Week 1" Is Defined
"See you in week 34" sounds simple until two calendars disagree about which week that is. The ISO standard settles it with one clear rule. Here is the rule, and the odd cases it creates.
The ISO 8601 rules
ISO 8601 is the international standard for dates and times. It defines week numbers with three rules:
- Weeks start on Monday and end on Sunday.
- Week 1 is the week that contains the first Thursday of the year.
- Equivalent to (2): week 1 is the week that contains 4 January, since 4 January is always in the week with the year's first Thursday.
A year has 52 or 53 weeks. Every week belongs wholly to one ISO year, never split between two.
Why week 1 can start in December
Because the rule is based on Thursday, a week counts toward the year that has the majority of its days, which means at least four out of seven. If 1 January falls on a Friday, Saturday or Sunday, those days belong to the last week of the previous year. If it falls on a Monday to Thursday, that week is week 1 of the new year, and it may begin on 29, 30 or 31 December.
- 1 January 2026 is a Thursday, so week 1 of 2026 starts on Monday 29 December 2025.
- 1 January 2027 is a Friday, so it belongs to the last week of 2026; week 1 of 2027 starts on Monday 4 January.
This is why the ISO "week-year" can differ from the calendar year for a few days at the start and end of the year. Software that mixes the two is a well-known source of bugs, such as a report showing "week 1 of 2025" for 30 December 2024.
Week 53
Most years have 52 weeks; some have 53. A year has 53 ISO weeks if 1 January is a Thursday, or if it is a Wednesday in a leap year. Years with 53 weeks in recent times include 2015, 2020 and 2026; the next after that is 2032. If you plan by week number, weeks-per-year isn't fixed, so check before assuming.
Other week-numbering systems
- United States and Canada (common practice): weeks often start on Sunday, and week 1 is the week containing 1 January. So week numbers can differ from ISO ones by one.
- Middle East: some calendars start the week on Saturday.
- Excel's WEEKNUM: uses the US system by default. Use
ISOWEEKNUMto get the ISO week.
If you work across countries, state which system you mean, or write the ISO form: 2026-W34, or 2026-W34-3 for the Wednesday of that week.
Where week numbers are used
- Logistics and manufacturing: delivery and production schedules across Europe and Asia.
- Project planning and sprints: many teams schedule by week number.
- Payroll and school calendars: especially in Scandinavia, Germany and the Netherlands.
- Date codes on products: a "2534" code is often week 34 of 2025.
Working out the week number by hand
- Find the date's ordinal day in the year (1 January = 1).
- Find its weekday number (Monday = 1 … Sunday = 7).
- Compute
(ordinal − weekday + 10) ÷ 7and take the whole number. - If the result is 0, the date belongs to the last week of the previous year; if it's 53 but the year has only 52 weeks, it belongs to week 1 of the next.
Example: 15 August 2026 is ordinal day 227 and a Saturday (6). (227 − 6 + 10) ÷ 7 = 33, so it's week 33.
Frequently asked questions
What week number is it now?
The calendar tool highlights today's date and shows its ISO week number.
Why does my calendar show a different week number?
It probably uses the US system, with Sunday as the start of the week and week 1 containing 1 January, rather than ISO.
Can a year have 54 weeks?
Not in ISO 8601. Years have 52 or 53 weeks. The US system can produce week 54 in some years.
How do I write a week in ISO format?
Year, "W", and a two-digit week: 2026-W07.