Week Number Calculator
The current ISO week number is shown at the top. Look up the week of any date, find the Monday-to-Sunday dates of any week number, or browse all the weeks of a year.
What week is a date?
Dates of a week number
All weeks of the year
Week numbers follow ISO 8601 (weeks start on Monday, week 1 contains 4 January), the standard used in Europe and most of the world. The US (Sunday start) week number is shown for comparison.
How to use the Week Number Calculator
- Read the current week in the green panel at the top. It updates automatically at midnight.
- Check a date: pick any date under What week is a date? to see its week number and the Monday–Sunday range of that week.
- Find a week's dates: enter a week number and a year, then press Calculate dates.
- Browse a year: choose a year in All weeks of the year to see every week with its start and end date, and press Download CSV to save it.
The ISO 8601 week rules
- Weeks start on Monday and end on Sunday.
- Week 1 is the week that contains 4 January (equivalently, the first week with a Thursday in the new year).
- Each week belongs to the year that contains its Thursday. That is why 29–31 December can be week 1 of the next year, and 1–3 January can be week 52 or 53 of the previous year.
- A date can be written as
YYYY-Www-D, for example2026-W43-1for Monday 19 October 2026.
Worked example
To find the week of Friday 1 January 2027, take the Thursday of the same Monday–Sunday week: Thursday 31 December 2026. That Thursday is in 2026, so the date belongs to week-numbering year 2026. Thursday 31 December 2026 is day 365 of the year, so the week is ⌊(365 − 1) / 7⌋ + 1 = week 53. Thursday 1 January 2026, on the other hand, is in its own year, so 1 January 2026 is in week 1 of 2026, a week that starts on Monday 29 December 2025.
Start of week 1 and number of weeks
| Year | Week 1 starts | Last week ends | ISO weeks |
|---|---|---|---|
| 2024 | Mon 1 Jan 2024 | Sun 29 Dec 2024 | 52 |
| 2025 | Mon 30 Dec 2024 | Sun 28 Dec 2025 | 52 |
| 2026 | Mon 29 Dec 2025 | Sun 3 Jan 2027 | 53 |
| 2027 | Mon 4 Jan 2027 | Sun 2 Jan 2028 | 52 |
| 2028 | Mon 3 Jan 2028 | Sun 31 Dec 2028 | 52 |
Years with 53 ISO weeks in this century include 2004, 2009, 2015, 2020, 2026, 2032, 2037 and 2043.
Where week numbers are used
Week numbers are common in Europe and Scandinavia for planning ("delivery in week 12"), in manufacturing and retail calendars, payroll, school timetables and agile sprint planning. When sharing a week number with people in other countries, add the dates of the week as well, because the US system can give a different number for the same date.
Frequently asked questions
What week is it today?
The large number at the top of this page is today's week number according to ISO 8601, calculated from your device's date. Below it you can see the exact Monday-to-Sunday dates of the current week.
How are ISO week numbers calculated?
ISO 8601 weeks start on Monday. Week 1 is the week that contains 4 January, which is the same as the week with the year's first Thursday. Every week belongs to the year that contains its Thursday, so a few days at the start or end of a year can belong to the previous or next year's week numbering.
Why is 1 January sometimes in week 53 or week 52?
If 1 January falls on a Friday, Saturday or Sunday, its week has more days in the previous year, so it belongs to the last week of that year. For example, Friday 1 January 2027 is in week 53 of 2026. If 1 January falls on Monday to Thursday, it is in week 1.
How many weeks are in a year?
Most years have 52 ISO weeks. A year has 53 weeks if it starts on a Thursday, or if it is a leap year that starts on a Wednesday. 2020 and 2026 have 53 weeks; 2027 has 52.
Is the US week number different?
Often, yes. In the US system (used by default in Excel's WEEKNUM function) weeks start on Sunday and week 1 is the week that contains 1 January, so the number can be one higher than the ISO week, and a year can have 54 partial weeks. Europe, ISO-based business software and Excel's ISOWEEKNUM use the ISO system.
How do I get the ISO week number in Excel or Google Sheets?
Use =ISOWEEKNUM(A1) in Excel or Google Sheets, where A1 contains a date. =WEEKNUM(A1, 21) gives the same result in Excel. The plain =WEEKNUM(A1) uses the US Sunday-start system.