Week Number Calculator
Find the ISO 8601 week number for any date, or look up the dates covered by a given week number. Includes day of year and quarter.
Last reviewed: 30 July 2026
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How ISO week numbers work
ISO 8601 defines week 1 as the week containing the first Thursday of the year. Because weeks always run Monday to Sunday and are never split across a year boundary, this rule guarantees that week 1 always has at least four of its days in the new year.
An equivalent and often easier test: week 1 is always the week containing 4 January. It is also the first week with a majority of its days in the new year.
Why some dates belong to a different year
The last few days of December can fall into week 1 of the following year, and the first few days of January can fall into week 52 or 53 of the previous one. That is why the result shows a week-numbering year separately from the calendar year. When recording an ISO week date, always write both, in the form 2026-W31.
52-week and 53-week years
Most years contain 52 ISO weeks. A year has 53 when it begins on a Thursday, or when it is a leap year beginning on a Wednesday. Businesses using 4-4-5 accounting calendars have to plan for the occasional 53rd week, which is why the result tells you how many the year contains.
Worked example
Thursday 1 January 2026
- 1 January 2026 is a Thursday, so its week contains the first Thursday of the year.
- That makes it week 1 of 2026, running Monday 29 December 2025 to Sunday 4 January 2026.
- 29, 30 and 31 December 2025 therefore belong to week 1 of week-numbering year 2026.
Because 2026 starts on a Thursday it is a 53-week year, ending with week 53 on Sunday 3 January 2027.
Limitations
- Only the ISO 8601 convention is supported. North American systems that start weeks on Sunday will give different numbers.
- Requesting week 53 of a year that only has 52 weeks rolls into the following year, and the result says so.
Frequently asked questions
What makes a week "week 1"?
Under ISO 8601, week 1 is the week containing the first Thursday of the year — equivalently, the week containing 4 January. Weeks always run Monday to Sunday, and every week belongs entirely to one year.
Why does 1 January sometimes show a week number from the previous year?
Because a week is never split between two years. If 1 January falls on a Friday, Saturday or Sunday, that week has fewer than four days in the new year, so the whole week counts as the last week of the previous year. 1 January 2021 was a Friday and fell in week 53 of 2020.
How many weeks are there in a year?
Either 52 or 53. A year has 53 ISO weeks when it starts on a Thursday, or when it is a leap year starting on a Wednesday. 2026 has 53 weeks; 2025 has 52.
Is this the same as the week number my company or spreadsheet uses?
Not necessarily. Some systems start weeks on a Sunday, and some define week 1 as the week containing 1 January regardless of how many days fall in the new year. This calculator uses ISO 8601, which is the international standard and the one used across most of Europe. Check which convention your system uses before relying on a match.
What is the week-numbering year?
It is the year the week belongs to, which can differ from the calendar year of the date itself. 30 December 2019 sits in week 1 of week-numbering year 2020. Writing an ISO week date as 2020-W01-1 avoids the ambiguity.