Week number of the year
Which week of the year a date falls in, under ISO 8601, which is the standard businesses use and does not always match what you would guess.
Your figures
Result
- ISO year it belongs to
- 2,026
- Falls on a
- Thursday
- Day of the year
- 239
- Days left in the year
- 126
- Year elapsed
- 65.5%
- Quarter
- 3
- Leap year?
- comun.no
What we assume
- ISO 8601 applies: the week starts on Monday and week 1 is the one containing the first Thursday of the year.
- That is why the first days of January can belong to the last week of the previous year, and the other way round.
- An ISO year can have 52 or 53 weeks.
How it is calculated
Numbering weeks looks like it should be trivial. It is not. ISO 8601 sets two rules:
- The week starts on Monday, not Sunday.
- Week 1 is the one containing the first Thursday of the year, which is the same as saying the first week with at least four days in the new year.
The odd consequence is that a date in January can belong to the previous ISO year. 1 January 2027 falls on a Friday, so its week has only three days in 2027: it belongs to week 53 of 2026. When that happens, we tell you.
Why it matters
Plenty of companies plan by week number: "we ship in week 34". If your system uses the US convention (week starts Sunday, week 1 contains 1 January) the numbers do not line up, and that is where a whole week of misunderstanding comes from.
An example
15 June 2026 is a Monday and falls in week 25. It is day 166 of the year, 199 are left, and you are in the second quarter with 45.5% of the year gone.
Frequently asked questions
Why does my calendar say a different week?
Because there is more than one convention. The US one starts the week on Sunday and numbers as week 1 the one containing 1 January, which can give a different number. We use ISO 8601, the international standard and the one Europe uses.
Can there be a week 53?
Yes. It happens when the year starts on a Thursday, or on a Wednesday in a leap year. Roughly every five or six years.
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Updated on 2026-08-26. Calculations run in your browser; nothing you type is sent to a server.