Week Number Calculator — ISO Week of Any Date
Pick a date and see its ISO week number, the week-numbering year, and the Monday-to-Sunday range that week covers.
ISO week — of —
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What this tool does
It converts a calendar date into its ISO 8601 week number — the 1-to-53 count of
weeks within a year — together with the week-numbering year and the exact Monday-to-Sunday range
the week covers. Week numbers are used for planning, payroll and logistics, and appear in tools
such as Excel’s ISOWEEKNUM, so it helps to be able to check any date by hand.
How ISO week numbers are defined
The ISO week is simple once you know the rule: weeks start on Monday and end on Sunday, and week 1 is the week that contains the first Thursday of the year. Equivalently, week 1 is the week containing 4 January. Because the anchor is a weekday rather than 1 January, the ISO week-numbering year can differ from the calendar year near the boundaries: the last days of December often belong to week 1 of the following year, and the first days of January to week 52 or 53 of the preceding one.
A year has 52 ISO weeks in most cases, but 53 when the year begins on a Thursday, or is a leap year that begins on a Wednesday. The week number here is computed directly from the date, so the 52-versus-53 case is handled automatically.
Examples
| Date | ISO week | Week range |
|---|---|---|
| 2026-01-01 (Thu) | Week 1 of 2026 | 2025-12-29 → 2026-01-04 |
| 2025-12-29 (Mon) | Week 1 of 2026 | 2025-12-29 → 2026-01-04 |
| 2021-01-01 (Fri) | Week 53 of 2020 | 2020-12-28 → 2021-01-03 |
Frequently asked questions
What is a week number?
It is the ordinal count of a week within a year, from 1 to 52 or 53. The most widely used definition is the ISO 8601 week, which is what this tool reports: weeks run from Monday to Sunday, and week 1 is the week that contains the year’s first Thursday.
Why can a date belong to a different year than it looks?
Because ISO weeks are anchored to the Thursday they contain. Late December can fall in week 1 of the next year, and early January can fall in week 52 or 53 of the previous one. That is why the tool reports an ISO week-numbering year separately from the calendar date.
How many weeks are in a year?
A year has 52 weeks most of the time, but 53 when it starts on a Thursday, or is a leap year starting on a Wednesday. For example 2020 had 53 ISO weeks. The count depends on the ISO week-numbering year, not the calendar year.
Which countries use week numbers?
ISO week numbers are the standard in most of Europe, in business and logistics planning, and in spreadsheet functions such as ISOWEEKNUM (Excel) and WEEKNUM. In the United States weeks are often counted from Sunday instead, so the number can differ by one.
When does week 1 start?
Week 1 is the week containing 4 January, equivalently the week containing the first Thursday of the year. In 2026 that week runs from Monday 29 December 2025 to Sunday 4 January 2026.
Is anything sent to a server?
No. The calculation is plain calendar arithmetic in your browser.
Go deeper
Longer explanations of the problems this tool solves:
Sources
- ISO 8601, Date and time format — the week-numbering rule used here (week 1 contains the first Thursday).
- Microsoft, WEEKNUM / ISOWEEKNUM function reference — the spreadsheet implementation of the same standard. support.microsoft.com
Everything runs in your browser — nothing you type is uploaded.