date ↔ ISO week
Week number calculator
1 January 2026 is in ISO week 1 of 2026, written 2026-W01-4, because that week holds the first Thursday of the year. Enter a date to see its week, or switch to enter a week and see its dates.
- 2026-01-01 = 2026-W01-4
- 2027-01-01 = 2026-W53-5
- 2024-12-30 = 2025-W01-1
Weeks follow ISO 8601: they start on Monday and week 1 holds the first Thursday. The US week number, which starts on Sunday, is shown beside it for comparison. Today's date is the starting value.
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At a glance
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- Not applicable — you type a date, nothing is sent anywhere
- Standard
- ISO 8601: weeks start on Monday, week 1 holds the first Thursday
- Year length
- 52 or 53 weeks
- US week
- Shown for comparison: starts on Sunday, week 1 holds 1 January
ISO weeks by year, and ISO against US numbering
These tables are computed by the same functions as the calculator. Weekday numbers run from 1 (Monday) to 7 (Sunday).
| Year | Weeks | Week 1 starts on | Last week ends on | Weekday of 1 January |
|---|---|---|---|---|
| 2024 | 52 | 2024-01-01 | 2024-12-29 | 1 |
| 2025 | 52 | 2024-12-30 | 2025-12-28 | 3 |
| 2026 | 53 | 2025-12-29 | 2027-01-03 | 4 |
| 2027 | 52 | 2027-01-04 | 2028-01-02 | 5 |
| 2028 | 52 | 2028-01-03 | 2028-12-31 | 6 |
| 2029 | 52 | 2029-01-01 | 2029-12-30 | 1 |
| 2030 | 52 | 2029-12-31 | 2030-12-29 | 2 |
| Date | ISO week date | US week number |
|---|---|---|
| 2024-01-01 | 2024-W01-1 | 1 |
| 2025-01-01 | 2025-W01-3 | 1 |
| 2026-01-01 | 2026-W01-4 | 1 |
| 2027-01-01 | 2026-W53-5 | 1 |
| 2028-01-01 | 2027-W52-6 | 1 |
How to find a week number
Enter a date
Use the date picker or type the date as YYYY-MM-DD, for example 2026-12-31. It starts as today's date on your device's clock.
Read the week
You see the ISO week number, the full label such as 2026-W53-4, the weekday, the Monday and Sunday that enclose the week, the day of the year and the quarter.
Or go from week to dates
Switch to week to dates, type a year and a week number, and get that week's Monday and Sunday. A week number that the year does not have, such as week 53 of 2025, is refused.
How ISO week numbers work
ISO 8601 numbers weeks from Monday to Sunday. Week 1 of a year is the week that holds the first Thursday, which is the same as the week that holds 4 January. A year has 52 or 53 such weeks: 53 when 1 January is a Thursday, or a Wednesday in a leap year. The years 2015, 2020, 2026 and 2032 have 53 weeks. Because of this, the first days of January can belong to the last week of the year before, and the last days of December to week 1 of the next year: 1 January 2027 is in week 53 of 2026, and 30 December 2024 is in week 1 of 2025.
The US convention is different, not wrong: weeks start on Sunday and week 1 is the week that holds 1 January, even if it has only one day in it. Spreadsheets and calendars may offer either, so the same date can have two week numbers. The tool shows the ISO number as the answer and the US number next to it. All counting is done on calendar dates in UTC, years 1 to 9999. Some countries and companies number weeks in other ways; this calculator follows ISO 8601 only.
Week number questions
What week number is it today?
Open the page: the date field starts as today's date on your device and the week number appears at once. For example, 28 October 2026 is in ISO week 44 of 2026, and it is day 301 of the year.
Is week 1 the week with January 1?
Not in ISO 8601. There, week 1 is the week with the first Thursday, which always holds 4 January. In the US convention, week 1 is the week with 1 January and weeks start on Sunday. So 1 January 2027 is week 1 in the US system, but week 53 of 2026 in ISO.
How many weeks are in a year?
An ISO year has 52 or 53 weeks. It has 53 when 1 January is a Thursday, or when the year is a leap year and 1 January is a Wednesday. 2026 has 53 weeks, and 2024, 2025 and 2027 have 52.
Why does 1 January sometimes fall in week 52 or 53?
A week belongs to the year that holds most of its days, which is the year of its Thursday. If 1 January falls on a Friday, Saturday or Sunday, its week has its Thursday in the old year. 1 January 2027 is a Friday, so it is in week 53 of 2026.
What are the dates of a given week?
Switch to week to dates, enter the year and the week number, and read the Monday and the Sunday. Week 1 of 2026 runs from 2025-12-29 to 2026-01-04, and week 53 of 2026 runs from 2026-12-28 to 2027-01-03.
What does 2026-W01-4 mean?
It is the ISO week date format: the week-numbering year 2026, week 01, and weekday 4, where Monday is 1 and Sunday is 7. So 2026-W01-4 is Thursday, 1 January 2026.