ISO week numbers: how they work and why 2026 has 53 weeks
Weeks run Monday to Sunday, week 1 is the week with the year’s first Thursday, and that is why a year can have 52 or 53 weeks.
An ISO week number tells you which week of the year a date is in, following the international standard ISO 8601. Weeks start on Monday. Week 1 is the week that contains the year’s first Thursday, which is the same as the week containing January 4. Every week belongs wholly to one year, so the last days of December can be in week 1 of the next year and the first days of January in week 52 or 53 of the previous one.
Why 2026 has 53 weeks
A year has 52 weeks and one or two days, so the extra days add up and, every five or six years, there is room for a 53rd week. Under the ISO rule that happens when the year starts on a Thursday, or on a Wednesday in a leap year. 2026 starts on a Thursday, so it has 53 weeks: week 53 runs from Monday, December 28, 2026 to Sunday, January 3, 2027.
Years with 53 ISO weeks from 2020 to 2040: 2020, 2026, 2032, 2037.
When week 1 starts
| Year | Week 1 starts | Last week ends | Weeks |
|---|---|---|---|
| 2025 | Monday, December 30, 2024 | Sunday, December 28, 2025 | 52 |
| 2026 | Monday, December 29, 2025 | Sunday, January 3, 2027 | 53 |
| 2027 | Monday, January 4, 2027 | Sunday, January 2, 2028 | 52 |
| 2028 | Monday, January 3, 2028 | Sunday, December 31, 2028 | 52 |
| 2029 | Monday, January 1, 2029 | Sunday, December 30, 2029 | 52 |
| 2030 | Monday, December 31, 2029 | Sunday, December 29, 2030 | 52 |
Other ways of numbering weeks
Not every system uses the ISO rule. Some US software, including the default of Excel’s WEEKNUM function, counts the week containing January 1 as week 1 and starts weeks on Sunday, so its numbers can be one higher than ISO numbers for part of the year. Excel’s ISOWEEKNUM function and most European diaries and planning tools use the ISO rule. Check which one your workplace uses before you plan a rota by week number.
Week numbers on your calendar
Tick “Week numbers (ISO 8601)” in the calendar maker to print the number beside each row of a month. Week numbers line up best on a Monday-start calendar; on a Sunday-start calendar each row is labelled with the ISO week of its Monday. See week start by country.