What this tool does
ISO 8601 week numbers look like a small detail and are not. 2026 has 53 ISO weeks rather than 52, because 1 January 2026 fell on a Thursday, so a payroll, a reporting period or a weekly dashboard that assumes 52 weeks is short by a full working week this year. This tool gives the ISO week of any date, converts a week number back into a date, and shows the two rival conventions disagreeing with it.
How it works
Weeks start on Monday, and week 1 is the week containing the year's first Thursday, which is the same as the week containing 4 January. Every Thursday therefore sits in week 1 of its own year and every week contains exactly one Thursday, and that single property is what makes the numbering agree between two people who have never spoken. It is also why 1 January can belong to the previous ISO year: 1 January 2022 was a Saturday, so it is 2021-W52.
An ISO year has 53 weeks when 1 January falls on a Thursday, or on a Wednesday in a leap year. In 2026 the first case applies, so week 1 began on Monday 29 December 2025 and week 53 runs from Monday 28 December 2026 into January 2027. ISO 2026 therefore ends after the calendar year does, and anything that divides a year by 52 to get a weekly rate is wrong for this year.
The other two systems are shown beside it, because three week systems give three answers for the same date. The US convention counts from the Sunday of the week containing 1 January, so 1 January 2022 is week 1 of 2022 where ISO calls it 2021-W52, and it can produce a 53rd week in years where ISO has 52. The Monday-first variant used by some libraries starts week 1 at the first Monday of the year and calls the days before it week 0. Where the systems happen to agree on your date, that is exactly why the disagreement is easy to miss.
Everything runs on plain day numbers with no clock and no timezone, so a daylight saving transition near local midnight cannot shift the day underneath you. The tool also converts back: give it a year, a week and a weekday and it returns the date, rolling a week number past the end of its ISO year into week 1 of the next instead of quietly pointing at a week that does not exist.
Worked example
Which ISO week contains 28 September 2026, and how long is ISO 2026?
- ISO week 1 of 2026 is the week containing 4 January 2026, which began on Monday 29 December 2025
- 28 September 2026 is a Monday, so its week runs from Monday 28 September to Sunday 4 October 2026, which is W40 of ISO 2026
- 1 January 2026 was a Thursday, which is the case that produces a 53-week ISO year
- Week 53 therefore runs from Monday 28 December 2026 to Sunday 3 January 2027
28 September 2026 is W40 of ISO 2026, and ISO 2026 has 53 weeks that run past the calendar year into January 2027.
Accuracy and limitations
- A hard-coded 52-week assumption in payroll, reporting or a dashboard is wrong for 2026. The number of ISO weeks in any year can be checked here, and that check is worth repeating each January.
- Because the ISO week-year diverges from the calendar year within a few days of both boundaries, sorting dates as text by a yyyy-Www string can put adjacent dates out of order: 31 December 2021, 1 January 2022 and 2 January 2022 are all 2021-W52.
- This is the ISO 8601 convention. If your contract, payroll software or reporting pack specifies the US or Monday-first convention instead, that is the one to use, and the tool shows all three so you can see which one you are looking at.
Frequently asked questions
- How many ISO weeks are in 2026?
- ISO 2026 has 53 weeks, not 52. An ISO year has 53 weeks when 1 January falls on a Thursday, or on a Wednesday in a leap year, and 1 January 2026 was a Thursday. Week 1 began on Monday 29 December 2025 and week 53 runs into January 2027.
- What is the ISO week number rule?
- Weeks start on Monday, and week 1 is the week containing the year's first Thursday, which is the same as the week containing 4 January. Every week then contains exactly one Thursday, and that single rule is what makes two people in different systems agree without ever discussing it.
- Why is 1 January sometimes in the previous year?
- Because the ISO week-year follows the week rather than the calendar year, and week 1 is the week containing the first Thursday. 1 January 2022 was a Saturday, so it falls in the week that began on 27 December 2021 and is therefore 2021-W52, not 2022-W01.
- How does the US week number differ?
- The US convention starts weeks on Sunday and treats the week containing 1 January as week 1, counting partial weeks. On 1 January 2022 that gives week 1 of 2022 where ISO gives 2021-W52, and the system can produce a 53rd week in years where ISO has only 52.
- Can I convert a week number back into a date?
- Yes, give the ISO year, the week number and the weekday. A week number past the end of its ISO year, such as week 53 of a 52-week year, is treated as the numbering running past the year end and rolls into week 1 of the next year rather than pointing at a week that does not exist.