ISO Week Numbers: Why Some Years Have 53 Weeks
October 21, 2026
The ISO 8601 standard numbers weeks so that every week starts on Monday and week 1 is the week containing the year's first Thursday (equivalently, the week containing January 4). A year has 52 or 53 such weeks. 2026 has 53 because January 1 falls on a Thursday; 2025 and 2027 have 52.
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Because week 1 holds the first Thursday, it always has at least four days in the new year. If January 1 is a Friday, Saturday, or Sunday, those days belong to the last week of the previous year. For example, 2026 starts on a Thursday, so its week 1 begins on Monday, December 29, 2025.
A year has 53 weeks if January 1 is a Thursday, or if it is a leap year and January 1 is a Wednesday. Otherwise it has 52.
| Year | ISO weeks | Reason |
|---|---|---|
| 2020 | 53 | Leap year starting Wednesday |
| 2025 | 52 | Starts on Wednesday, not a leap year |
| 2026 | 53 | Starts on Thursday |
| 2027 | 52 | Starts on Friday |
| 2032 | 53 | Leap year starting Thursday |
Why It Matters
Businesses that plan by week, such as retailers, manufacturers, and payroll departments, depend on week numbers. A 53-week year means an extra week of sales or payroll, which can distort year-over-year comparisons unless adjusted for.
The ISO week-year can differ from the calendar year for a few days. December 29 to 31 may be in week 1 of the next year, and January 1 to 3 may be in week 52 or 53 of the previous year. Software that mixes the two causes classic New Year bugs.
Other Week Systems
The US commonly starts weeks on Sunday and calls the week containing January 1 week 1, which gives different numbers from ISO. Spreadsheets provide both: WEEKNUM uses the US-style default, while ISOWEEKNUM follows the standard. State which system you use when sharing week numbers across teams or countries.
Frequently Asked Questions
Which day does an ISO week start on?
Monday.
Which years have 53 ISO weeks?
Years in which January 1 is a Thursday, or a leap year in which January 1 is a Wednesday.
Can January 1 be in week 52 or 53?
Yes, if it falls on a Friday, Saturday, or Sunday.
What is the Excel function for ISO week number?
ISOWEEKNUM.