📅 Week Number Calculator
Find the ISO week number for any date, with the dates the week starts and ends, the day of the year and the days remaining.
Quick answer: ISO weeks start on Monday and week 1 is the week containing the year’s first Thursday. So January 1, 2026 (a Thursday) is in week 1 of 2026, running Monday, December 29, 2025 to Sunday, January 4, 2026, and it is day 1 of 365.
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Week Number Calculator inputs
Result
ISO week number for Sunday, October 11, 2026
Week 41
2026-W41-7
| Week starts (Monday) | Monday, October 5, 2026 |
| Week ends (Sunday) | Sunday, October 11, 2026 |
| ISO week-numbering year | 2026 |
- Day of the year
- 284
- Days left in year
- 81
- US week number (Sun start)
- 42
- ISO weeks in 2026
- 53
Days of ISO week 41, 2026
| Day | Date | Day of year |
|---|---|---|
| Monday | 2026-10-05 | 278 |
| Tuesday | 2026-10-06 | 279 |
| Wednesday | 2026-10-07 | 280 |
| Thursday | 2026-10-08 | 281 |
| Friday | 2026-10-09 | 282 |
| Saturday | 2026-10-10 | 283 |
| Sunday | 2026-10-11 | 284 |
How ISO week numbers work
The ISO 8601 standard, used across Europe and in most business software, defines weeks that start on Monday. Week 1 is the first week with at least four days in the new year – which is the same as the week containing the first Thursday. Every week belongs wholly to one "week-numbering year".
Week = ⌊(day of year of that week's Thursday − 1) ÷ 7⌋ + 1
To find it, move to the Thursday of the same Monday–Sunday week; that Thursday's year is the ISO year, and its day of the year gives the week.
Worked example
January 1, 2026 is a Thursday, so it is its own week's Thursday and day 1 of the year: ⌊0 ÷ 7⌋ + 1 = week 1. The week runs from Monday, December 29, 2025 to Sunday, January 4, 2026. 2026 has 365 days, so 364 remain after January 1.
Most of Europe, Asia and international business software follow the ISO rule shown here.
First ISO week of recent years
| Year | Week 1 starts | Weeks |
|---|---|---|
| 2025 | Monday, December 30, 2024 | 52 |
| 2026 | Monday, December 29, 2025 | 53 |
| 2027 | Monday, January 4, 2027 | 52 |
| 2028 | Monday, January 3, 2028 | 52 |
Tips
- Write ISO weeks as 2026-W01; add the weekday number (Monday = 1) for a full ISO week date, such as 2026-W01-4.
- Payroll and planning tools may use US-style Sunday weeks – check which system your organisation follows.
- Year boundaries: always quote the ISO year with the week, because late December can belong to week 1 of the next year.
Frequently asked questions
What is an ISO week number?
ISO 8601 numbers the weeks of the year from 1 to 52 or 53. Weeks run Monday to Sunday, and week 1 is the week that contains the first Thursday of January – equivalently, the week containing January 4.
Why can January 1 be in week 52 or 53?
If January 1 falls on a Friday, Saturday or Sunday, most of that week belongs to the previous year, so it is counted as the last week of the old year. For example January 1, 2027 (a Friday) is in week 53 of 2026.
Which years have 53 weeks?
A year has 53 ISO weeks when it starts on a Thursday, or when it is a leap year starting on a Wednesday. 2020 and 2026 have 53 weeks; 2032 is the next after that.
How is the US week number different?
Many US calendars start weeks on Sunday and call the week containing January 1 week 1, so the numbers can differ by one from ISO weeks. Both are shown in the results.